The number ‘9’ has been solved for six times in the puzzle, in all but 3 of the 3×3 blocks. In two out of three of these blocks, it is possible to solve for where the ‘9’ goes by eliminating places it can go by looking at the location of ‘9’s in 3×3 blocks in the corresponding row and column of this 3×3 block.
2) This is basically the opposite of 1. You start with rows, columns, or 3×3 blocks that are nearly solved, figure out which values are missing, then eliminate possibilities by looking at 3×3 blocks in the same row or column as the nearly solved 3×3 block that contains the missing values (and thus can eliminate possibilities in the nearly solved 3×3 block). An example
The top middle and bottom middle blocks of this sudoku are basically identical and nearly solved. In the top middle block the three unsolved values are 1, 4, and 9. Looking at the top left block’s 4 location will eliminate the 4 being in the top left and top right cells of the top middle 3×3 block, so the 4 must be in the middle position of the top middle 3×3 block. The same is true with the 6 for the bottom middle block after examining the bottom right block’s 6 position.