I'm not a developer, but my impression is it's more of a do-it-yourself environment.
The most up-to-date "tutorials" are all the free and open-source projects you can discover as you go back through the forums. If you go really far back, I think there are a couple of attempts at beginners guides (using Qt4, so, not even sure if they still work.).
The two main projects are KOReader (which spun off a neat libEink API - EDIT: FBInk, not libEink, sorry about that NiLuJe!Although "libEink" does have a nice ring to it...) and Plato (which sombody recently piggybacked off of to create a vnc client in Rust, for example). Vlasovsoft and XCSoar are other huge ones but I don't know how active they are.
Edit: Newer ones which don't yet support all devices are Inkbox and rmkit:
https://www.mobileread.com/forums/sh...d.php?t=337972
http://rmkit.dev/rmkit-comes-to-kobo/
There are also various small projects you can build off of.
Basically start browsing, and you'll get an idea, I'm sure.