How to solve this 9×9 logic puzzle

9×9 · severe · 24 steps

This 9×9 board rates as severe (difficulty score 1208). Solving it from scratch takes 34 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced — nothing here is a guess.

First, every open cell left in this region sits in row 1, so its marker has to land there — clearing every other open cell in the row.
Next, 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.
Then, together, two regions have open cells only in rows 8 and 9 — 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.
After that, together, two regions have open cells only in rows 6 and 7 — 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.
Now, every remaining candidate in this region touches row 2, column 8 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
From there, every remaining candidate in this region touches row 4, column 2 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Following that, marking row 1, column 7 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
First, every remaining candidate in this region touches row 2, column 9 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, marking row 2, column 1 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Then, marking row 2, column 2 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
After that, marking row 2, column 3 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Now, marking row 2, column 6 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
From there, marking row 2, column 7 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Following that, place a marker at row 2, column 4. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column 4, and its region.
First, place a marker at row 7, column 6. Region has exactly one open cell left. Placing here clears the rest of row 7, column 6, and its region. Placing here also clears its 1 touching neighbour.
Next, place a marker at row 6, column 3. Region has exactly one open cell left. Placing here clears the rest of row 6, column 3, and its region. Placing here also clears its 1 touching neighbour.
Then, place a marker at row 5, column 1. Region has exactly one open cell left. Placing here clears the rest of row 5, column 1, and its region.
After that, place a marker at row 9, column 5. Region has exactly one open cell left. Placing here clears the rest of row 9, column 5, and its region.
Now, place a marker at row 8, column 2. Region has exactly one open cell left.
From there, every open cell left in this region sits in row 4, so its marker has to land there — clearing every other open cell in the row.
Following that, every remaining candidate in this region touches row 4, column 8 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
First, place a marker at row 4, column 7. Region has exactly one open cell left. Placing here clears the rest of row 4, column 7, and its region. Placing here also clears its 1 touching neighbour.
Next, place a marker at row 3, column 9. Region has exactly one open cell left. Placing here clears the rest of row 3, column 9, and its region.
Finally, place a marker at row 1, column 8. Region has exactly one open cell left.

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