I agree it's pretty complicated. Although the general principle of 'definiteness' covers many/most cases, it is far from complete.

I found this page by the British Council to be quite comprehensive, but may still miss something (eg. 'go to school' vs 'go to the school')

https://learnenglish.britishcouncil.org/grammar/english-gram...

Do you know of some other resources that explain many grammatical points well?