The answer to your question(s) would be a resounding "neither", I'm afraid. I'll try to get a bit more in-depth for the 1648-1806 period of HRE history where I'm best informed in, but hopefully you'll get other answers expounding on the many centuries before that. I can already give you a tl;dr though, if you like, and that's "It's fiendishly complicated".

The political and structural framework of the HRE was the following: There was the German King (who often also wore the Imperial Roman Crown) who stood at the top of the feudal structure of the Empire. Every lower-ranking prince received his (in rare cases also: her) fief from the King's hands, and throughout the HRE's history every time a fief passed hands or a new King/Emperor (I'll simply say "Emperor" from now on since throughout the entire early modern era, every German King also styled himself Emperor) there was a corresponding ceremony where the fief-holder would ritually swear fealty to the Emperor. So, a singular and centralised state with a strong monarch on top, no?

Well, of course it was much more complicated than that. In practice, the Emperor being the feudal overlord was more a legal theorem than political reality, and while the aforementioned ceremonies continued to be followed, in most cases from the late 17th century on, the various German lords sent representatives instead of appearing themselves as a subtle sign of defiance towards Imperial claims. Also, the Emperor was basically alone, as he had no governmental bureaucracy to his side...

...instead that this isn't the whole truth, either. Emperor Maximilian I, who ruled at the turn from the Middle Ages to the Early Modern, was very ambitious in his plans to centralise the Empire and create a functioning bureaucracy. For various reasons (the continuing resistance of the various princes being the central one), his plans didn't fully pan out and could only be realised in part. The most important achievements of Maximilian on this front were probably the establishment of an imperial court system, with the Imperial Chamber Court and the Imperial Aulic Council serving as the supreme appellation court in the Empire. But even there, we have to qualify that statement, since over the centuries many German territories tried (and often succeeded) in securing the privilege of their subjects not being able to appeal to the Imperial judiciary. Another enduring legacy of Maximilian's was the introduction of "Imperial districts" (Reichskreise), which ultimately got tasked with organising and supervising the mint, collecting the imperial taxes (well, tax - there was only one, and it didn't exactly constitute a lot of income), gathering and training Imperial Army troops and enforcing sentences of the Imperial judiciary. There also were a number of other, minor tasks, but it's more important to note that there were vast differences between the various districts; some of them were highly active, whereas others mostly existed on paper.

But there was an Imperial Judiciary and an Imperial Army so it can't have been too decentralised, right? Well, yes... and no. The two imperial courts had overlapping jurisdictions and sometimes worked openly against each other, the aforementioned privilegia de non appellando weakened the Imperial judiciary especially within the strongest individual member states, and the Imperial Chamber Court was notoriously underfunded; many of the cases brought before it took literal centuries to finally be resolved. Some never were, because the HRE's end in 1806 got in the way (although to be fair, those century-long cases also tended to be the politically most charged ones. In less exciting cases, the Court could be astonishingly quick, even!). The army was provided by the individual member states who generally speaking weren't all that inclined to supply the Emperor with a powerful standing military. In addition to that, there was no centralised Imperial Army doctrine or drill and the various units never exercised together, which in turn meant that their efficacy in actual warfare wasn't always a given.

On the other side of the scarce Imperial bureaucracy, there were the many (many, many) individual territories, who in themselves were highly varied and distinct. They were organised in the Imperial Diet (Reichstag), a mammoth parliament that until the 17th century would only come together sporadically. In 1663, the Reichstag met again in Regensburg, but when its members couldn't reach a consensus in several highly important questions they simply deferred the official end of the session (also because some princes feared that due to those debates the Emperor would refuse to call for another session) until perpetuity, creating the "Perpetual Diet" in the process which legally speaking was just one long, never-ending session of the Imperial Diet. This parliament didn't represent all of the Empire, however, since not every ruler in the Empire was allowed a seat (I'll tell more about them later on), and because Bohemia and Imperial Italy weren't represented either for a variety of reasons which all eventually end at "they were arguably no longer a part of the Empire, anyway".

The minor princes that were part of the Diet were divided in various groups. At the top were the prince-electors, a group of seven powerful princes and bishops whose most important job and privilege was to elect the Emperor. Who exactly was part of this elite group changed slightly over the centuries, and towards the end of the Empire the prince-electoral college even expanded to ten members. One of the prince-electors, the Archbishop of Mainz, also had the office of "Imperial Arch-chancellor of Germany" - in theory he led the imperial chancellery (in practice this was done by the "Imperial Vice Chancellor") and, most importantly, directed the sessions of the Imperial Diet. His prince-elector colleagues, the Archbishops of Cologne and Trier, also were Arch-chancellors of Italy and Burgundy, respectively, but those titles had long since lost virtually all of their practical power. The prince-electors even used to come together for parliamentary sessions of their own where they debated various imperial matters, but those mostly ceased with the Thirty Years' War.

Below the prince-electors came the Imperial Princes and Prelates. Those included powerful princes who didn't make it into the prince-electoral college, but also most Catholic bishops of the Empire and also all those abbots and abbesses who had the distinction of leading an "Imperial Abbey". This group had about 100 seats altogether, but six of those seats were "curial votes", i.e. one single vote cast together by a larger number of Diet members who didn't make the cut for a vote of their own. If I didn't miscount there were 166 of those unlucky princes and prelates.

At the lowest rung of the ladder were the Imperial cities, i.e. city-states (some of them very big and influential, others tiny) who weren't part of any larger territory, answered directly to the Emperor and took part at the Imperial Diet. Towards the end of the Empire there were 51 Imperial cities who had a whopping two votes inbetween them.