“This is what it is and how it works. Now it's up to you to figure out what you can do with it.”
Just a little note: I call an Amstrad CPC an Amstrad CPC (or CPC for short) and an Amstrad Plus an Amstrad Plus (or Plus for short). No “CPC old” or “CPC+” here.
The firmware is the native Operating System of the Amstrad CPC/Plus. Reading these documents is highly recommanded.