Q:

How the actions work in BuddyPress? 

I am working with BuddyPress; and, there are many actions, such as: do_action('bp_before_wrapper'), do_action( 'bp_before_bodywrap' ), do_action( 'bp_before_container' )...; bu,  I didn't find where define such actions. 
Are they default actions in BuddyPress?  

A:

do_action is a WordPress call.  There may be actions hooked to a do_action or it may just be there so developers can call it if they want.  
For example do_action( 'bp_before_container' ); is not hooked to anything, but you could write a function that hooks to it.  
function snail_content() {
    echo 'I am a snail';
}
add_action('bp_before_container', 'snail_content' );

Read the WP codex re do_action. 
btw - do_action( 'bp_before_container' ); is in bp-themes which is deprecated.  Use the files in bp-templates. 