That's all true, though there's a key difference: 99% of nobles got there by being born into it, and their qualifications are random and very few of them had to work for it. The percentage of high ranking corpers is lower, depending on how much nepotism is allowed in the society. The percentage in the military is often even lower, at least in a modern military. In a pre-19th century military where you bought a commission, obviously that's not the case, though there are always legacies even in modern times. We had a young Lt on our ship whose godfather was a 2-star in the same region, and our skipper's tirades skipped over him and crucified everyone nearby, so I can't say there's nothing, though it's definitely not the same as in 1815, when you bought your rank from the crown.
This part, insofar as a player, is concerned varies. There's two ways, at least when I do it, a player can be a noble. The first is they roll it up initially. So, they start out as a noble. That requires a backstory as to who they're related to, have fealty to, etc. They're stuck with that position. That is, they are who their family is. They also may or may not be the person to inherit the title.
The second way is they are either promoted as a noble, or promoted into nobility. Promoted nobles who were initially nobles automatically become the person that inherited this new, higher title. They are still stuck with their backstory however. Just because they became more powerful in terms of their noble position doesn't change who they're aligned and opposed to.
The second sort is a character that becomes a noble in the generation process. This sort has gotten a new title, usually knight, and they hold the title. They then have to determine who granted them this title and what their faction in politics, etc., is as a result. That is, they become aligned to certain higher nobles and are now in the crosshairs of opposed noble families, at least to some extent.
I also allow nobles to buy a commission. Pricey, but doable. You can't be a complete putz however as a character, even nobles have standards... I also tend to give nobility far more cash and such on mustering out simply because they're going to need it. Being a noble is never cheap, and the cash won't last. It's simply a bridge until the character either succeeds or fails on their own.
Of course, this varies some with what world, polity, etc., you are rolling up in. But it is true for the 3I and for most of their client states.
As for corporations and such, they have a sort of nobility that runs in parallel with the political nobility. Here, characters that would be nobles can either be corporate, political, or both. A corporate noble gets a different title (example: Magnate, the equivalent of a baron) to denote that their title is a corporate / economic / business one. They have the same sort of backstory only their alliances and rivalries are in the business world primarily, not in politics. There is some crossover, but it isn't as clear cut as being a political noble.
A character would have to be a merchant or have another appropriate role to be in that line of nobility. Merchant / corporate nobility run business and aren't normally part of the Moot or politics. If you get high enough corporate-wise, then the two start to merge and the difference vanishes.
Academics and clergy, likewise, have something similar to nobility in their ranks. So, if you were an academic and a noble you might be the equivalent of a Nobel Prize winner and have a title that goes with that award making you a noble in terms of status. Your alliances and rivalries are in the field of academics, and you are unlikely to ever advance high enough to get into politics. Organized religion is the same way. Get high enough and you are the equivalent of a noble and that brings the same things to the table. Again, you are unlikely to ever rise to a point where you are also political unless there is a close connection between the church and state, and there are some polities in the game where that is the case just not the 3I.
With the military, nobility comes with success or when a noble chooses to go into the military. In my version, nobility initially rolled up automatically means you start out as an officer. Your success is on you, however. If you get socially promoted into nobility, it is for doing something that warrants that and that becomes part of your backstory.
