Every row, column and box must hold each digit once.
Start with a row, column or box that already has many digits, and see which are missing. Then pick one digit and check where it can still go in each row. When unsure, write candidates as notes. No guessing is needed: every puzzle here can be solved by logic alone.
In hints
When stuck, tap "Hint". It first gives a direction, then marks the board and explains the next step.