Where are you sourcing all your puzzles?

I remember the blocker to a Sudoku app I was making in secondary school was just getting good puzzles. They're hard to make, particularly if you're signing up to make a new one every day. I guess you could create them with AI now, but you'd run significant risk of them being uncalibrated for difficulty or just outright invalid.

I always thought people generated sudokus with heuristics, e.g. the less numbers remain, the harder it is.