如何在没有composer的情况下使用PHP库？

This is a really noob question... How do I use PHP libraries downloaded from GitHub? I've never experienced using libraries (even with the use of Composer). Where do I put the library folder? Are there any other steps I need to take to make the library work?

I'm trying to use PHPSpreadsheet to read and write Excel files; in their GitHub documentation, it's only about using the library in code.

I've tried searching for ways on how to use libraries without the use of Composer but they were all really specific and didn't apply to my situation. There were answers about using libraries on a server and about using libraries on Wordpress.

I just need to use the library for my thesis which will just be on one PC, no servers or anything.