The development of the role of the scholar bureaucrat changed drastically over time and as such, the role of a scholar in the Tang Dynasty was drastically different from the Song dynasty (and was a constantly evolving role.) During the Tang Dynasty, the imperial examination and the status it conferred did not meaningfully limit the power of the nobility within China. At its height, perhaps 10% of government officials had passed through the Imperial examination system and many of those were themselves from prominent noble families. Thus, don't think of the scholar-gentry as a supplantation of the landed aristocracy of the Tang dynasty; think of it as an addition. Also, logically speaking, it should come as no surprise that those who were wealthy and privileged tended to have more opportunities to study and do well on the examinations. While in theory, it was a meritocratic opportunity, in practice it was not. This is of course ignoring that when the advantages of wealth and privilege didn't do, plenty of corruption, bribery, threats, etc. would suffice in place.

It's in the Song dynasty that we see a significant shift and the empowerment of this scholar-gentry class. Even then, at its height, a majority of government officials were not recruited from the imperial examinations and because of the nature of this system, those that successfully passed the system typically came from wealthy families that had also passed the exam. This would be more accurately described as the supplantation of the previous government structure for recruitment of officials in favour of the imperial examination, though even then, the imperial examination by virtue of its format and the needs of those who would take it, would heavily favour those who came from wealthy and landed families.

Also, I think you have a misconception of what the nobility of China looked like. Chinese noble families did not necessarily have titles and the weight of a title was different in China than it was in feudal Europe at the time. To give a random example, Li Jiao, a chancellor of the Tang Dynasty (/Zhou Dynasty) was born to an influential noble family (the Li clan of Zhao Prefecture), and was given the title "Duke of Zhao" during the reign of emperor Zhongzong. His father did not have this title. He did not inherit this title, but his family would have been undeniably considered a "noble family". Indeed, later members of this clan who would also go on to become chancellors such as Li Jue, would have their own title (Duke Zhenmu of Zanhuang) which you'll note similarly was not inherited and was not a title he would pass on either. A third member of this aristocratic clan, Li Deyu was similarly granted the title "Duke of Wei" which, as you guessed, he did not inherit and was granted. Though in this case, his father was also a chancellor and held a Ducal honour as well.

So as you can see, the understanding and value of being a "duke" in an imperial Chinese court was drastically different than in a feudal European society. Whereas the title was typically hereditary and conferred real power on the wielder in countries like France or England, the understanding of the title in Tang Imperial contexts was hardly the same. In fact, I'd wager a guess that practically any chancellor within the Tang court held some type of noble title (probably duke) regardless of their origins. Also, another thing to remember is that "noble families" in the Chinese sense (or clans) were much, much larger than European ones.

Hopefully that helped give a brief overview. Let me know if you want any clarification though.

I'll add on to what u/doneanddead wrote by pointing out a broader pattern of push and pull between "great families" (at various times, 世家 shijia or 門閥 menfa) and "men of service" (士 shi) up until Song times, when the universal exam system ended the distinction between the two and a new, hybrid elite came into being, the "scholar-gentry" (绅士 shenshi).

From at least the early Zhou period, most positions of power were held on a hereditary basis by families of independent wealth and prestige- not only the thrones of the feudal states but also many military and civil offices were the preserve of specific noble lineages. For these lineages, political office was their due on the grounds of their prestigious ancestry as well as their private wealth (in the form of lands and dependents) and military power (in the form of their privately-held horses, arms, armor and chariots).

However these great lineages also naturally produced junior branches over time, forming a lower aristocracy which had more modest means but still had access to the education befitting noblemen. Enter the "men of service," who lacked hereditary claim to office and made their case for entry into state service on the grounds of their specialized technical, ritual, or cultural expertise. Where the "great families" were mainly interested in the state as a means to legitimize and protect what they already had, this lower rung saw opportunities for advancement and enrichment through a government career, and the more idealistic among them also saw the state as a tool for social change rather than just a guardian of the established social order.

As the Zhou dynasty declined and the Warring States kingdoms began to form, a combination of factors led to a dramatic rise for the shi and near extinction for the Zhou nobility. The expansion of a handful of powerful states at the expense of all others dispossessed most of the former lordly and ministerial families, and other prominent clans were destroyed in warfare or domestic conspiracies (the Partition of Jin being a particularly bloody example). Meanwhile state capacity was expanding rapidly and men of service flocked to the many new administrative offices created during this time. Land reform turned many former clients into independent smallholders, and direct taxation by the state superceded the tribute formerly paid to the nobility. Lastly in states like Wei and Qin, ruling houses became increasingly hostile towards their ministerial families and, seeing the men of service as less politically threatening, used them to strip the high nobility of much of their privileges. Commoners who reached the necessary intellectual and cultural attainments were now also able to occasionally enter the ranks of the men of service.

The uprising which brought down the Qin empire was a sort of revenge of the high nobility, especially the great houses of Chu who had never been cowed as the nobles of many other states had been. Xiang Yu, scion of a line of hereditary Chu generals, and his envisioned "Eighteen Kingdoms" ruled by what remained of the former royal and ministerial houses, embodied this reaction. He was ultimately defeated by Liu Bang, a minor bureaucrat of peasant ancestry- which is to say, a man of service- and the Han dynasty sealed the fate of the old Zhou nobility, with feudal titles becoming a privilege that could be given or revoked by the Emperor at will.

However, a new batch of great families emerged over the course of the Han dynasty, marked by landowning on a grand scale and by proximity to the Emperor through a mix of high office holding, intermarriage with the imperial clan, and estates in or near the capital region. The usurper Wang Mang came from one such family. The trend accelerated under the Eastern Han, with many provincial landlord families joining in on the monopolization of office by the very wealthy and connected. The men of service, in the sense of relative outsiders who sought office through educational attainment or "merit," were sidelined by the great families of the capital and the provinces. This was partly a consequence of economic factors like the increased concentration of land in the hands of major landlords, but also political factors like the central government's gradual retreat from intensive management of land and people, and the ability of high officials to stuff lower offices with their relatives and clients through the system of recommendation.

The great families once again became the real holders of power during the collapse of the Han and maintained their grip as effective central authority mostly disappeared through the Western Jin and the Northern and Southern Dynasties. A kind of re-feudalization occurred as sons of officials gained a right to enter government office at a rank determined by the highest position achieved by their fathers. The free peasantry fell into ever greater dependency on the great families and serfdom became widespread. And especially in the North, great families once again became a warrior elite with hereditary military obligations, reinforcing their claims to hereditary political power. In these regards they were quite similar to the former Zhou high nobility. The great families of Sui and Tang, including the imperial clans themselves, originated from this semi-feudal Northern social elite. Although they reopened the examinations to produce a slow trickle of outsider talent, both dynasties relied on the aristocrats for the great majority of important offices.

The great innovation of the Song was to put all candidates for office through the same set of formal exams. By ending hereditary officeholding (as practiced in Wei, Jin, and the Northern and Southern dynasties), closing the path to office via recommendation (i.e., through patronage), and making the exam system universal (making the kind of education prized by the shi into a standard requirement), the Song prevented the emergence of power blocs on the scale of those seen in Zhou times or from the later Han period onwards. It became much less common for a single line to produce many consecutive generations of office holders, and the ruling class became broader and more economically level. In the context of the prosperity and higher living standards of the Song period, this also meant a higher degree of upward mobility than in previous dynasties, even if the exams still favored the leisure class overall (who could afford to devote their youth to study instead of labor). This is the point where the "scholar-gentry" emerges, as a kind of fusion between the former great families and men of service. Like the men of service, they relied on their intellectual and cultural credentials and had no birthright claims to office, but they were more economically and socially secure and could prosper even when only intermittently drawing state salaries. This hybrid elite was the dominant social force in China through to the late Qing.