hello i would like to ask where do i find the version for 1.21

If you specifically want 1.21 you'll have to build it yourself, otherwise you can use CI builds.

is there a vidwo how to do it

Check out https://github.com/cabaletta/baritone/blob/1.21.1/SETUP.md#build-it-yourself, it's a document that explains how to build the mod from source code into a .jar-file