I downloaded a library from GitHub and it won’t show up. I downloaded it from here https://github.com/mkeller0815/65xx_Library.
Where did you place the downloaded library?
Where are you looking for the downloaded library?
What version of Kicad are you using?
What is your OS?
You will find the answer to the last two if you go to Kicad help / About Kicad / Top RH corner and copy / paste.
KiCad does not search for libraries. You have to list each library you want to use in their library table.
You can do this with
Project Manager / Preferences / Manage Symbol Libraries and
Project Manager / Preferences / Manage Footprint Libraries
Both these tables have a global and a project specific variant, and you should copy or move the files first to a logical location before you add a link to your library.
Try tracing where it got downloaded to. You might have been checking the wrong placed. You can also do a search on your PC to make it quicker.