Well a good start would be to put the downloaded library into the lib folder so that you have lib/<your new library>/<source files>.
To control the build options of the library, and which files are build if exclusions are needed, can be done through adding a library.json file:
https://docs.platformio.org/en/latest/manifests/library-json/index.html
More complicated libraries might need custom Python logic to build, (see extra_scripts above), but that really depends on the library. A good library.json should work for most libraries.