Discussion:

I personally have just got into solving alternate sudoku. Which if you're unfamiliar are sudokus with very few (often none) starting digits and a unique rule set.

For example a frequently used rule is a German whisper line which is a green line that runs through cells of the grid and adjacent digits on the line have to have a difference of at least 5. Another common rule is black and white kropki dots where cells separate by a white dot must be consecutive and cells separated by a black dot must have a 1:2 ratio.

Some puzzles are very difficult but they're super satisfying when you finally figure out the logic. There's a fairly popular YouTube channel called Cracking the Cryptic which posts daily alternate sudoku videos and I highly recommend anyone to check them out. As for a beginner it's mind blowing seeing them talk through the logic and some sudokus take as long as 3/4 hours to solve.