I've always had the notion that the gentility could "rent" opera(theater) boxes or in a more roundabout way sponsor the venue and get a box assigned that way. They would have to pay a considerable annual fee and in return nobody else could use their box. They themselves (and selected family friends) could use the box to watch as many performances as they like without additional fees. Having a box was a status symbol and prestigious.
Sort of like a Royal/State Box but available to anybody with money. Or how companies today might buy the luxury boxes of Sport Venues.
When I tried to look this up online I couldn’t really find anything, except the Royal box. Was I looking in the wrong places? Or was this not a thing that happened?
You've got it right! Opera subscribers would have their own assigned box, and they could do whatever they wanted and bring whoever they wanted there, and it would be theirs night after night after night, for every performance. (It was normal to watch operas multiple times in a row, and the performers would mix the music up with new interpretations of their areas.) We have some interesting material culture of opera around this, which are old opera "tickets" that have survived. When you had a box you would have a subscriber's coin thing that you showed at the door to get in, not a paper ticket. The coin would have your name and box number on it. Other people in the loose areas would pay at the door and not have any sort of ticket, because there was no assigned seating (or assigned standing, in some places.) In some places you could also have a subscription with pass coin to the loose areas, and just get there early to get a good seat. There was a custom of some people sending their servants in early to save them a good seat. A lot of these seem to have survived and didn't get melted down for things, I think they were just popular little things to keep.
A bone example from the early 19th century
Whole bunch of them in the British Museum
Apparently Elizabeth Taylor had a tacky necklace of them
And of course, in the middle of the season, when everyone had already seen the premiere and it wasn't time for the close of that opera, and it was hot that day, and no one in the family was feeling it, those boxes would just sit empty.
Why it went away - the rise of the middle class and the decline of the nobility in the late 19th and early twentieth century. There were less people who had the money and leisure time to see opera every night and therefore having a box for the whole season, but there were lots of rising class people who wanted to get them some of that high class art consumption, but only once in a while and on their schedule. So they started renting out some boxes for the evening, less and less people saw any value in having them the whole season, and eventually we now just have us each essentially renting a single seat for a night.
If you're interested in this sort of thing, I highly recommend The Gilded Stage: A Social History of Opera, which gives this particular argument for why it went away.