when interacting with scintilla control the Scintilla documentation is a good starting point.

The concept is 99% like this, you send a message (the SCI_…) and either you get an immediate
return or/and you will be notified by the so called notifications.

The remainig 1% is it doesn’t work as expected. ;-)

When interacting with gui aka notepad++ then you need to get an understanding
how windows applications work in general. The main topic here is windows messages.

Cheers
Claudia