Some important development docs are found in the wiki: https://dev.minetest.net/

Notable pages:

• Releasing Minetest
• Engine translations
• Changelog
• Organisation
• Code style guidelines

• 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.