In this example, the opening has the following properties:
For 2D representation: a block consisting of three lines

For 3D representation: a block consisting of an arched doorway, created by making a polyline (see Polyline) and extruding it (see Profile Objects).

A value was assigned for Height, so that the block will be inserted above the floor.
Remember, the blocks have to be created in advance, and are assigned to the opening in the Opening Properties.

If necessary, change the workplane back to By World. You may have changed the workplane while creating one of the blocks, but the workplane must be along the bottom of the walls.
Activate Insert Opening, and make sure Anchor to Wall is selected. (Anchor to Roof is similar, except that openings will tilt themselves to fit along a sloped roof.)
Drag the opening to one of the walls. The block is dragged by its reference point, which can be changed if needed. See Editing a Block.

The block aligns itself to the wall, and cuts the opening.

Drag more blocks onto the walls. The block will change its alignment to fit any wall.

Note: This tool only creates rectangular cutouts, even if the block is non-rectangular. You can change this by modifying the block. See Edit Opening Modifier.
In Plan view, the openings are represented by the 2D block.

If you assign the 3D block a material such as glass (in the 3D page of its Properties), and assign a brick material for the walls, you can see the results in render mode.

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