BuddyPress follows exactly the same plugin API as WordPress. You’ll find that there are hundreds of actions and filters within BuddyPress that will allow you to hook into and modify existing functionality.
By using the plugin API, you can avoid modifying core BuddyPress files allowing you to upgrade BuddyPress smoothly and not lose your existing changes. It really is a bad idea to modify core files, if you want to extend and modify BuddyPress, you should try your hardest to learn how the plugin API works.
A good place to get started is the WordPress plugin API documentation. This will explain to you how actions and filters work, and how you should properly use them. BuddyPress follows exactly the same principles. In fact, BuddyPress uses this API to integrate into WordPress, not a single core modification is needed. That should tell you all you need to know about the power of the plugin API.