How to solve this 9×9 logic puzzle

9×9 · sharp · 16 steps

This 9×9 board rates as sharp (difficulty score 988). Solving it from scratch takes 27 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Paired regions, Forced chain. Every step below is forced — nothing here is a guess.

First, every open cell left in this region sits in column 1, so its marker has to land there — clearing every other open cell in the column.
Next, together, two regions have open cells only in rows 1 and 2 — with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
Then, place a marker at row 3, column 2. Region has exactly one open cell left. Placing here clears the rest of row 3, column 2, and its region. Placing here also clears its 1 touching neighbour.
After that, place a marker at row 5, column 3. Region has exactly one open cell left. Placing here clears the rest of row 5, column 3, and its region. Placing here also clears its 2 touching neighbours.
Now, every open cell left in this region sits in column 5, so its marker has to land there — clearing every other open cell in the column.
From there, every open cell in column 4 belongs to the same region, so that region's marker has to be somewhere in this column — clearing every other open cell in the region.
Following that, marking row 1, column 7 immediately breaks another row, column, or region, so it can't be the marker there.
First, marking row 1, column 8 immediately breaks another row, column, or region, so it can't be the marker there.
Next, marking row 1, column 9 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Then, place a marker at row 2, column 8. Region has exactly one open cell left. Placing here clears the rest of row 2, column 8, and its region.
After that, place a marker at row 1, column 6. Region has exactly one open cell left. Placing here clears the rest of row 1, column 6, and its region.
Now, place a marker at row 4, column 9. Region has exactly one open cell left. Placing here clears the rest of row 4, column 9, and its region.
From there, place a marker at row 6, column 7. Region has exactly one open cell left. Placing here clears the rest of row 6, column 7, and its region.
Following that, place a marker at row 7, column 5. Region has exactly one open cell left. Placing here clears the rest of row 7, column 5, and its region. Placing here also clears its 1 touching neighbour.
First, place a marker at row 9, column 4. Region has exactly one open cell left. Placing here clears the rest of row 9, column 4, and its region.
Finally, place a marker at row 8, column 1. Region has exactly one open cell left.

The key move here was Forced chain — once you can spot that, this board falls quickly. ∎