One of the most annoying things any creator has had to deal with is copyright. Things are always messy, and even after years of streamlining it is far from perfect. Even if you are given permission to use a work by the copyright holder, a company, or even an individual can swoop in and steal profits or even take down your project. To put it simply, it's complicated, especially when debated in courts, and even across countries.
In general, a copyright law is just a protection over a work you've made, entitling that you own any original work you make, whether it's published or unpublished. One type of work, relating precisely to media, is what makes this so much more complicated, music. Although an artist can create music and generally own it, depending on the deal they have with a record label, they may or may not hold the copyright to it.
Aside from this basic principle, there also exists free use and attributions. Some licenses allow you to use a work freely, so long as the original author is credited, and you don't edit the original; while other uses require the work to not be monetized. Free use laws also fall under this, as they allow you to freely use a copyrighted work so long as you do not claim it as yours, and add your own creative input to warrant its use. Having learned about this in class and by reading terms of service for models and YouTube, things certainly don't seem at all complicated, as just reading a bit helps unsure fair use. However, social media and services convolute everything.
One such example is in YouTube, where even if a work (more typically songs) fall under fair use, you can still receive copyright strikes, and have your channel deleted, as YouTube doesn't follow copyright laws, but it's own policies that prevent content stealing. This has happened with movie and show reviewers who used footage of what they were critiquing (something that falls under fair use), who have their channels deleted after a company sends claims to YouTube. More typically this can happen when a critique of something is unfavorable o the company, and so in an attempt to get rid of negative PR, they delete unfavorable content, on grounds that they don't fall under fair use. The same can be used with songs, as some companies allow music to be used in the background, so long as the music alone isn't part of the content. Even then, streamers and video makers can have content deleted for playing a song in the background of a game or other content (without the intent of it). This has gone to extreme lengths, as sometimes even humming or playing a song that sounds too similar to another one can result in banning as bots automatically pick up on it. Even if you manage to get through all that legal mumbo-jumbo, your content can still be claimed, even if it is not copyrighted, as extorters can make a claim based on absolutely nothing, meaning anyone can do it.
But why is this relevant? Because music. As I want to include and maybe publish my work, I have to make sure it won't be taken down. As such, I've found websites that compile music and scores that can be used by others, without as much danger as just using the first song I see.
Aside from websites such as Dova-Syndrome, Chosic, Purple Planet, and even YouTube's own audio library, among other paid and free services, I have browsed a few to find fitting music for my project.
I'm certainly no legal advisor, so here's some more info on copyright! https://www.baylor.edu/copyright/index