Number Square strategy
Each row, column and box holds every digit exactly once. Check these three patterns, easiest first.
The last gap
When a row, column or box has one empty square, it takes the one digit it is missing.
Every digit appears exactly once in each group, so the missing digit has nowhere else to go.
The only digit that fits
When a square's row, column and box together already hold every digit but one, it takes that digit.
A digit can't repeat in any of the three groups a square sits in, so the digits already there are ruled out.
The only place for a digit
When a digit still has to go in a row, column or box and only one empty square there can take it, it goes there.
The group must hold the digit somewhere. Every other square is blocked by a copy of that digit in its own row or column.