Musenet takes an "intro" and then "evolves" it with AI in different styles such as Beatles, Mozart or Bluegrass. Interestingly you can upload your own custom midi track and let it evolve that too.
I tried adding a file created by Computoser as a starting point, and I think it made it more Beatles-y but it's hard to tell. Doesn't have a download option.
This wonderful tune creates interesting chord progressions for you. I tend to choose piano, turn off drums and set the bpm to around 80.
You have to pay to download the midi, but there are ways around this, using tools that turn recorded audio into midi. Although this may introduce errors or inaccuracies, we will live with these happy accidents.
YouTube has bots that check your videos to make sure you haven't used a copyrighted music track in your video. All these tracks are free to use on YouTube, Facebook, Instagram & Twitch without copyright issues.
This guide shows you how to download tracks from the Audio Library.
There are also lots of sound effects to download too.
Pay special attention to attribution requirements, with any audio user ever.