Why are obscenely rich Russian people "oligarchs" and other obscenely rich people "billionaires"?
Here is an explanation that may help you.
https://www.thestreet.com/markets/emerging-markets/what-exactly-is-a-russian-oligarch
Reading that, I can't see a massive difference. Corrupt people becoming fabulously wealthy through siphoning public money into private hands. Is Abramovich more corrupt than the Glazers? I don't recall the precise details, but when they "bought" Manchester United they somehow succeeded in transferring the debt they incurred in the deal onto the club itself, which, as I understood it at the time, meant that they had a free football club and quite a lot of millions in their bank account, and, in the event of the football club going under (that didnt seem likely since it was Manchester United) they Glazers personally wouldn't have been liable for the debt. (I'm happy to be corrected on these points. I haven't bothered to go back and check, but that is what I remember from however long ago it was). And of course, we have seen the financial probity demonstrated by the Tory party in this country, firstly in being prepared to take huge sums of laundered money from rich Russians (you don't have to be Russian to apply...) and secondly how they handed out covid-related contracts to their mates in unlawful ways.
Why are obscenely rich Russian people "oligarchs" and other obscenely rich people "billionaires"?
Because oligarchy is a feature of current Russian society. It's not a uniquely Russian feature nor a modern one, of course; I recently came across a reference to oligarchs in the context of 17th and 18th century minor German states (in a book written in 2013). Some of the patrons of Bach and Handel were probably oligarchs!
I take the point re vocabulary, but I still don't see much difference.
I doubt that Bach received a lot of cash from oligarchs because he really only had the one employer - Thomaskirche in Leipzig. I don't think he was ever especially wealthy. Handel, on the other hand, was well in with English royalty and was very rich indeed.
Mods: feel free to move bits to POBI, but you know the old saying "You can't keep politics out of sport..."