An old friend from university is an arranger and assistant for two well-known composers. He gets a basic (low) salary from each job at the expense of royalties (everything created by the assistants goes out in the name of the ‘head’ composer), but in the studio downtime he’s allowed to do his own work and this has led to a few arrangements for adverts and small indie films under his own name. Best of these is an advert for a fashion house which he re-licenses back to them every year on a 12 month contract for around £10k a time.
Looked up the accounts of a few other people I vaguely know who do music for a living to see how they’ve been getting on.
One person I was at uni with sang on a theme tune for a TV show almost 10 years ago (didn’t write it though as far as I’m aware) - still gets hundreds of thousands of streams every month. Accounts show they’ve been taking £25-45k a year since and appear to have a very healthy cash balance to draw from at similar levels for a long while to come. They were signed to a major label for a short time, released two albums which almost broke the top 50, rarely played live and hasn’t done anything since 2017. Seems likely they get their money predominantly from that one song.
An ex-colleague is doing really well. Signed to a major label while I was working with them, quit a short while later to record a debut album and go on tour. First album went top 10 and the second top 30, gets approx. 200,000 streams a month. Appears on the odd TV show, does a few festivals most summers with the occasional headline gig. Last couple of years their accounts say they took dividends of £85k and £170k. Guessing this is mostly from touring.