Music Theory

Hi folks, I wanted to ask you something I have been dealing with for as long as I have learned music theory: when is the point that you simply learn stuff by heart?

For example: I could not imagine writing out all keys in order to learn which notes a contained in them. But I learned all notes in all diatonic 7th chords by simply using flashcards. How am I to „practices“ my way into knowing what notes makes up a Dminor7?

Whats your experience with that? Where do you draw the line?

Whether its learning or practicing, what resources do you enjoy using online? My favorite for simple exercises is still https://www.teoria.com/. The simplicity of the website is just great