SecretFedSpy

Not OP, but it is very likely not a 'real' choir that you hear in trailer cues. Often times the deadlines for a custom trailer cue is very short so there isn't time to gather a full choir into a studio and record them. There likely isn't the budget either, although that certainly varies quite a bit. If the cue is written ahead of time and licensed (and not written specifically for that trailer) then it was likely written by a freelance musician who probably doesn't have the time or budget to hire a choir for each of his/her cues. Often times a freelance licensing focused musician will write many many cues in an effort to have a large number of cues in various libraries (not always the case) and it's not practical to hire a professional choir for any/all of them.

There are a number of very capable choir sample libraries that sound very convincing with a bit of elbow grease. EWQL Symphonic Choirs, SoundIron Venus/Mars choirs, 8dio Requiem and Strezov Sampling Storm Choir 2 are just a few examples of professional grade choir sample libraries.

Movie soundtracks are a completely different game. It is very likely that any A or B level movie will do everything they can to have live musicians play the score as opposed to using sample libraries.

As someone who is working towards a full time career writing music to be licensed (not just in trailers, but that's certainly one area) do you have any advice? It's a difficult industry to break into and requires you to be good at a variety of different skills. I'm more than a tad bit overwhelmed at the task ahead of me.

Will do. Thanks for the advice!