How to solve this 6×6 logic puzzle

6×6 · sharp · 15 steps

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This 6×6 board rates as sharp (difficulty score 1002). Solving it from scratch takes 21 logical steps, using 8 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

First, together, two regions have open cells only in columns E and F; 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.
Next, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
Then, every remaining candidate in this region touches B2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, every remaining candidate in this region touches C4 and C5 and E5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Now, marking A1 would force F2 (the last open cell in region 2), then E6 (the last open cell in region 6), and then column C would be left with no open cell for its marker. So A1 can't be the marker there.
From there, every open cell in column A belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
Following that, if B1 were the marker, it would force F2 (the last open cell in region 2), then C6 (the last open cell in region 4), and column E would end up with no open cell for its marker, which can't happen. So it can't go there.
First, every open cell left in this region sits in column C, so its marker has to land there; that clears every other open cell in the column.
Next, every remaining candidate in this region touches A3 and C3 and A4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Then, place a marker at B3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column B, and its region. Placing here also clears its 2 touching neighbours.
After that, place a marker at C1. Region has exactly one open cell left. Placing here clears the rest of row 1, column C, and its region.
Now, place a marker at F2. Region has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region.
From there, place a marker at E6. Region has exactly one open cell left. Placing here clears the rest of row 6, column E, and its region. Placing here also clears its 1 touching neighbour.
Following that, place a marker at D4. Region has exactly one open cell left.
Finally, place a marker at A5. Region has exactly one open cell left.

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