BuddyPress.org

Well, when yo call xprofile_template_loop_end, then this hook should only run once the complete loop has finished. There’s no way of adding stuff before or after a certain action hook (except when there’s another hook), only right where the hook is. One way of adding things to a loop is to use a counter for the loop and then add a dynamic hook to every entry, like so:
`do_action( ‘some_custom_hook_’. $counter );`

BuddyPress plugins are just WordPress plugins, so the best starting point is probably the WP codex:
https://codex.wordpress.org/Main_Page

Then there’s the skeleton component that does a pretty good job of explaining some BP specific stuff:
https://wordpress.org/extend/plugins/buddypress-skeleton-component/

And of course the BP plugin files and other BP plugins, that you can have a look through.