Jetbrains custom language plugin sources

Hi there!
I haven't been developing custom language plugins for IDEA for some time, but i guess it's time to return to the industry ; )
The last time i did develop though i clearly remember reading Jetbrain's own Ruby plugin sources almost as a manual to openAPI and not-so-openAPI as well. I managed to pick up lexer and parser automated testing technique, working with fancy completion, good caching, placing gutter marks and many more useful things.
However when i briefly scanned materials about plugin development now i haven't found anything alike.

So, what's the situation today:
can i get everything ruby/python/php/groovy etc plugins do by using openAPI?
are there opensource versions of the above plugins anywhere?
is there any cookbook for custom language plugins?

I'd be glad for any comments,
thank you