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 1025). Solving it from scratch takes 21 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, 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; that clears every other open cell in the row.
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 C2 and E2 and C3 and E3, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Now, every remaining candidate in this region touches E5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
From there, marking A1 would force F5 (the last open cell in row 5), and then column E would be left with no open cell for its marker. So A1 can't be the marker there.
Following that, that one can't be the marker: at C1 it would force D3 (the last open cell in region 2), then F2 (the last open cell in region 4), then A4 (the last open cell in region 3), and row 5 would be left with no open cell for its marker.
First, place a marker at B1. Region has exactly one open cell left. Placing here clears the rest of row 1, column B, and its region. Placing here also clears its 1 touching neighbour.
Next, every open cell left in this region sits in column A, so its marker has to land there; that clears every other open cell in the column.
Then, if F2 were the marker, it would force D3 (the last open cell in region 2), and row 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
After that, place a marker at D2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column D, and its region.
Now, place a marker at F3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, 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.
Following that, place a marker at A5. Region has exactly one open cell left. Placing here clears the rest of row 5, column A, and its region.
Finally, place a marker at C4. Region has exactly one open cell left.

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