In the 3D viewport header, immediately after the four viewport shading buttons. It shows a pause icon while the viewport is live, and a pressed play icon while it is paused.

Yes. Right-click the button and choose Assign Shortcut.

Geometry nodes, the whole modifier stack, physics and particle systems, materials and shading, animation and drivers. The number of objects Blender evaluates in the scene drops to zero.

Everywhere except the 3D view. The Properties editor, node editors, Outliner, timeline and spreadsheet all behave normally, and every change you make is recorded as usual — only the viewport result waits.

It does not make evaluation quicker — it stops it happening at all while you do not need to see the result. The gain is the waiting you skip, not a faster solve.