Some important development docs are found in the wiki: https://dev.minetest.net/
Notable pages:
- Developing minetestserver with Docker
- Android tips & tricks
- OS/library compatibility policy
- Miscellaneous
Oftentimes knowledge hasn't been written down (yet) and your best bet is to ask someone experienced and/or the core developers.
Feel free to join the #minetest-dev IRC and ask questions related to engine development.