How to solve this 6×6 logic puzzle

6×6 · sharp · 16 steps

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This 6×6 board rates as sharp (difficulty score 1091). Solving it from scratch takes 21 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Line confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

First, every open cell in row 1 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
Next, together, two regions have open cells only in columns F and E; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
Then, every open cell in row 6 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
After that, every remaining candidate in this region touches C3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Now, together, two regions have open cells only in rows 4 and 5; 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.
From there, if B1 were the marker, it would force A6 (the last open cell in column A), then C4 (the last open cell in column C), then D2 (the last open cell in region 2), and column E would end up with no open cell for its marker, which can't happen. So it can't go there.
Following that, that one can't be the marker: at C1 it would force F2 (the last open cell in row 2), then E4 (the last open cell in region 6), and row 5 would be left with no open cell for its marker.
First, if C2 were the marker, it would force A1 (the last open cell in region 1), then B6 (the last open cell in column B), then F5 (the last open cell in region 6), and the chain continues, and row 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
Next, that one can't be the marker: at D2 it would force A1 (the last open cell in region 1), then F3 (the last open cell in region 3), and row 5 would be left with no open cell for its marker.
Then, marking A1 would force F2 (the last open cell in row 2), then E4 (the last open cell in region 6), and then row 5 would be left with no open cell for its marker. So A1 can't be the marker there.
After that, place a marker at D1. Region has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region.
Now, place a marker at C4. Region has exactly one open cell left. Placing here clears the rest of row 4, column C, and its region. Placing here also clears its 1 touching neighbour.
From there, place a marker at B2. Region has exactly one open cell left. Placing here clears the rest of row 2, column B, and its region.
Following that, place a marker at F5. Region has exactly one open cell left. Placing here clears the rest of row 5, column F, and its region.
First, place a marker at E3. Region has exactly one open cell left.
Finally, place a marker at A6. Region has exactly one open cell left.

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