BuddyPress.org

First, read this webpage:

https://codex.wordpress.org/Plugin_API#Actions

That’ll explain the basics of hooks and filters. If you want to know which hooks are in BuddyPress, scan through the source code and look for do_action() functions as well as apply_filter() functions, those are your hooks, and you can “hook” into them using the functions add_action() and add_filter() respectively. I don’t think there’s too much documentation around for BuddyPress hooks, so it’s best to look into the file/function you want to extend and find your hooks.