liuzengqiang:
I meant IDE version 1.8.4. I know what board managers do but not sure how. I'm compiling arduino micro.
the only core in IDE installation folder is the version of AVR core bundled with IDE.
every other core or other version of the AVR core is installed with the boards package in Arduino15 folder in users home folder and it doesn't change with IDE installation.
and you asked about GitHub. all board packages have a GtHub repo
here are the cores by Arduino