While it's true that a lot of distinct markers of Manchu culture have been heavily eroded, including the language, that has not meant the end of Manchu identity. There are still over 10 million self-identifying Manchus in the People's Republic (thereby making up around 0.85% of the population and thus the third-largest ethnic group after Han and Zhuang), and at least as of the early 2010s, Manchu language programmes were going relatively strong. Nevertheless, you're not wrong to observe that Manchu culture isn't quite what it used to be.

And that isn't to say that there has been no Manchu influence on China. To answer your first sub-question, the Manchus infamously imposed the queue hairstyle for men, and Qing-era men's clothing was also heavily Manchu-influenced, as shirts that buttoned down the side (in Manchu style) came to coexist alongside the more Chinese front-buttoned style, while the clothing of higher-status men was standardised in Manchu style in the form of the changshan. While Han women's fashion tended to be largely a continuation of Ming styles during the Qing, after the Qing fell and their 'sumptuary laws' (for lack of a better word) became defunct, Manchu women's robes became adapted to produce the qipao (旗袍), literally 'Banner/Manchu dress' (this is also known by its transcription from Cantonese cheongsam 長衫 'long dress'). There are some limited loanwords from Manchu in northeastern Chinese as well. Perhaps one of the most significant influences have been culinary. In particular, the sweet pastry known as sacima/shaqima, which is now found across China proper, has Manchu origins.

The only real imposition out of all those, though, was men's fashion.

I'd expect them to force the Hans & other ethnic groups to sacrifice their own respective culture & assimilate to the culture of the Manchus, and also adopt the language of the Manchus. Because that's what most rulers/colonizers/conquerors all over the world do (did).

The thing is, no, that's not what most foreign rulers, colonisers and conquerors do, and I could probably name more foreign conquerors who adopted and adapted traditions of their conquests, or kept the spheres of conqueror and conquered separate, or failed to hybridise those spheres, than those who didn't. Alexander (in)famously 'Medised' after conquering the Achaemenid empire, adopting Persian clothing and encouraging use of the Persian language by Macedonian elites; the Mongols did not enforce the use of Mongolian among their federates or hold onto Tengriism, such that most of the former Mongol empire ended up primarily Turkic-speaking and Muslim; the Timurid princes who became the Mughal shahs did not enforce the speaking of Chaghatai Turkic in northern India, nor even Islam to a particularly significant extent; British conquest did not stop the Québécois from speaking French; the Soviets did not (successfully) stamp out the languages of Central Asia.

The Manchus ran their empire in a pluralistic fashion, with each major constituency generally retaining its traditional customs and practices (in appearance at least if not necessarily in substance), and without much attempt – at least, not after the initial post-conquest decades – to disrupt that to a particularly considerable extent. To try and enforce changes to language policy and so forth does not fit the overall pattern of Qing behaviour – hair and clothing policy for men notwithstanding.

But in that case, why did the Qing not manage to successfully protect Manchu cultural distinctiveness? That gets into a much deeper, longue durée discussion of Manchu identity and culture that is far beyond my ability to provide in full, but a quick, whistle-stop summary (based largely on the relatively pro-ethnicity perspective of Mark Elliott) would be:

• There were no Manchus before 1635, which is when Hong Taiji claimed that 'Jurchen' was a pejorative and that 'Manchu' was the true, pure term for the people whom he was about to arbitrarily define as 'Manchus', a designation which included mixed Jurchen-Mongol groups like the Yehenara clan and excluded some groups like the outer 'Wild Jurchen' tribes not yet under Manchu dominion. 'Jurchen' did not simply map onto 'Manchu', as the former's definition was always somewhat loose, whereas the latter was, initially at least, defined by arbitrary decision of the monarch.

• All Manchus came to be enrolled in the Eight Banners, an organisation that came to include some Mongols and many Han Chinese, partly from Ming and pre-Ming colonists in Manchuria and partly from defecting Ming troops in China proper. By the early eighteenth century, Han Chinese made up perhaps some 50% of the Banner population, compared to 38% Manchus and 12% Mongols. As such, the institution of the Banners didn't actually do much in terms of strengthening Manchu identity and culture. Moreover, while Manchus made up over half of the civil bureaucracy in China, these were concentrated in the metropolitan and high-level provincial governments, so in practice the low-level Manchu presence in the provinces was limited to garrisons in major cities, which had regulated but nevertheless close contact with local Han Chinese populations.

• In the mid-18th century, the Banners were reformed so that a large number of Han Bannermen were either disenrolled, or, in the case of those whose ancestors were settlers in Manchuria, redefined as Manchus. In other words, a sizeable proportion of 'Manchus' after 1750 or so – including today – were descended principally from Han Chinese ancestors. These reforms resulted in the numbers flipping a bit, with the ethnic proportions in the Banners by around 1911 being calculated at about 53.5% Manchu, 15.7% Mongol, and 31.8% Han. From the late eighteenth century on, 'Manchu' and 'Banner' also became increasingly synonymous, with the three 'ethnic' categories becoming more a matter of internal status rather than outright ethnic difference.

• The Qianlong Emperor did attempt to reinforce Manchu language education and the continuation of Manchu customs within the Banners – and the removal or recategorisation of Han Bannermen has been suggested as part of that plan – but was ultimately unsuccessful in doing so sustainably. In any case, the basis of Manchu ethnic identity was no longer rooted in the imperial ideal of Manchu practices, but rather the now more tightly knit Banner system, which maintained insular Banner communities in China's major cities – so insular in fact that they actually remained linguistically distinct from their Han Chinese neighbours, by virtue of speaking official Beijing dialect Mandarin rather than local Chinese varieties!

• The years between 1912 and 1927 saw the progressive loss of Banner stipends, a key concession made by the republican revolutionaries which had allowed the Banner system to remain in place in all but name. In the provinces, where the stipends stopped first and Manchus were lesser in number, there seems to have been a melting into the local population. In Beijing and Manchuria, where Manchus were certainly being out-populated but to a lesser extent, and where communities remained relatively more coherent, Manchus retained some degree of cultural and linguistic distinctiveness, but that there was erosion was clear. The Japanese tried to exploit latent Manchu nationalism in their construction of Manchukuo, but this was not, by and large, successful, and the People's Republic actively suppressed notions of ethnic identity and cultural diversity until near the end of the 20th century, from which point there has been a resurgence in Manchu self-identification and cultural rediscovery.