The documentation of phpBB is catastrofic, but there are some useful resources about specific topics often used in theme and extension development. Here they are randomly sorted.
Phpbb3 unexhausted event list
https://wiki.phpbb.com/Event_List#Template_Events
More events can be found in the source code. Just grep the code for trigger_event
Kickstart an extension
An extensive tutorial here https://area51.phpbb.com/docs/dev/3.1.x/extensions/tutorial_basics.html
DBAL or the query builder
https://wiki.phpbb.com/Using_the_phpBB3.0_DBAL
Creating new routes
https://area51.phpbb.com/docs/dev/3.1.x/extensions/tutorial_controllers.html
Basic helper functions
https://wiki.phpbb.com/Using_phpBB3%27s_Basic_Functions
Useful global constants
https://area51.phpbb.com/docs/30x/coding-guidelines.html#constants
Globally available template variables
https://wiki.phpbb.com/Global_Template_Variables