Selection tools · Layer Comp

Modifiers work as in Photoshop: Shift adds, Alt subtracts, Shift+Alt intersects (pixel selections only). On a Mac, read Cmd for Ctrl.

Tool	Key	How
Lasso	L	Drag around an area; releasing closes it
Polygonal Lasso	Shift+L	Click points; Enter or the first point closes
Rectangle / Ellipse Marquee	M / Shift+M	Drag; Ctrl makes it square or round
Magic Wand	W	Click
Color Range (live)	Shift+W	Sample colours; Enter saves a live selection
Selection Brush	B	Paint the selection; Shift+B paints the active layer's mask
Quick Selection	Alt+W	Brush over something; the selection grows to its edges
Select Object	O	Click an object in the render; selected exactly from Cryptomatte
Select All / Deselect / Invert	Ctrl+A / Ctrl+D / Ctrl+Shift+I
Transform Selection	Ctrl+T	Moves the selection, not the pixels
Refine Edge	Ctrl+Alt+R	Snaps a rough edge to hair and foliage
Pick Color	I	Pixel readout; picks into a fill layer or the active node
Before / After	\	The Viewer shows the render under the layers, and back

A selection is made of two parts, and the whole of it is outlined with marching ants:

Shapes: the lasso, polygonal lasso and marquees write a Mask datablock, CS Lasso. The shapes stay editable point by point in the Image Editor's Mask mode, and feather works. They are live.
Pixels: the wand, brushes, Quick Selection and loaded selections write a packed image, CS Wand, tinted red in the Image Editor. It is baked, but the add-on notices when the image under it changes (see Stale wand selections).

Each feeds a node of the same name in the compositor. Neither is ever linked for you.

After a shape, the editor stays in View mode so the hotkeys keep working. Switch to Mask mode to edit points; there, Blender's own mask keys win.
Invert on a shapes-only selection keeps it as shapes: it adds a frame-sized shape and flips every shape between add and subtract.
Transform Selection: G, R and S switch between move, rotate and scale; Ctrl snaps, Shift is precise.