I'm ok with answers which pertain to either early, high or late middle ages; I've read in the past that English 'fuedalism' was very different from French, so answers could be for one or the other (sweeping generalization that ignore vastly different time-periods and cultures wouldn't be that useful).

Now, while I don't necessarily push for the existence of fuedalism (it may or may not have existed, the validity of using the term to refer to something that is very nuanced and still being examined is up for question), but for the purposes of this question, I'm talking about the vassal-fief-homage relationship as is most often understood.

So from how I understand it, based on u/Miles_Sine_Castrum's excellent response to one of my old questions:

Barons was the general term for any person, who by either virtue of dynasty or personal favour, held a specific set of lands as granted by the king. Holing such lands (per baroniam) meant the ability to adminster justice via baronial court, the right to demand things like socage, merchet, corvee, etc.; this could also include the ability to parcel out a portion of this barony and create a new title and thus make his own vassal. For all intents and purposes, they owned these lands (and the serfs living in it), although technically the true owner was still the monarch.

These lands, the right to benefit and utilize them, along with the accompanying titles/signorage (which afforded rank and precedence), and the inherent services that the person must now render to the king (court duties, military levies of knights or scutage, taxes) is called the fief. A Baron's fief's territory is called a Barony (while the fief itself would be called a baronship). Therefore, a baron was any tenant-in-chief/vassal-in-chief.

Now, some Barons, due to being of even higher esteem by the monarch, are granted fiefs which encompass a larger privilege and responsibility—i.e. countships/earldoms or dukedoms. Such fiefs involved much larger swaths of land (shires/counties and duchies). However, this didn't translate to per baroniam tenure—a count did not have the same control over all the manors (and serfs) in his county the same way a baron did in his barony. Further, the barons who held baronies in that county or duchy did not automatically become his vassals—they were still the king's direct vassals. What the count or duke did have was a gubernatorial position—they administered and controlled this territory on behalf of the king, which I guess included promulgating the King's policies such as taxes, laws/decrees, administering his justice, etc.

Of course, a Count or Duke was still held his own baronies, of which he did have near-sovereignty; and for the purposes of convenience and custom, most times a Count was given a county in which most of his baronies were located (since a Count needed to have local infrastructure to do his duties).

So my question is thus—what were the powers of a magnate (count, duke, marquess) have over his territory? Was I correct in saying he could "skim off the top" when collecting taxes that go directly to the king (from other tenants-in-chief with baronies in his territory)? Was he also able to impose his own taxes on top of that (ergo a commoner rendered taxes to his baron, his count, and his king)? Could he create his titles/vassals from some land in his county that did not belong to his baronies or other noble's baronies? Did a count automatically become the commander of all the knights/militia raised from their county (regardless if the knight never swore fealty or paid homage to said count; as they were all technically serving the King)?

Now, I understand that what I'm describing might be construed as the ideal (and I think it's very Anglocentric). As far as I know, many counts and dukes would eventually consolidate their power (i.e. either by getting into bed with the barons who held baronies in his territory, or by outright making them submit and pay homage to him directly, or usurping said baronies by marriage or force) to the point that they did in fact 'own' all the land within his county's territory (not just a governor, but as a sovereign). And this led to the general paradigm shift wherein magnates operated as their own small kingdoms (and indeed, some gained independence once they grew powerful enough to make and enforce such a claim or be acknowledged as such).

So am I correct in my understanding? Or am I way off the mark?