BuddyPress.org

Hello,
I am learning WordPress and BuddyPress. I am beginner in programming but I already know a bit. I hope there is still support here and there will be someone who will give me information.
I am asking for information about the buddypress structure itself. In which way Buddypress does ‘activity / post-form.php’ link to the stream of personal activities?
I don’t mean the possibility of adding post-form by me because I know it. I mean how BuddyPress does it in its original form.
I can’t find this place, I’m still missing something. Please help