An overlay is an image or one PDF page placed behind the grid so you can trace it.
Add one
Open the overlay manager with overlay and use overlay import. JPEG, PNG and PDF work, and for a multi-page PDF you pick the page. Each layout holds one overlay.
Line it up
Position it to match the grid, then lower its opacity to about 30 to 40 percent so grid lines stay visible while you trace. Use grid draw for straight walls and pencil draw for the rest.
Manage
overlay show and overlay hide toggle it without deleting. overlay remove deletes it for good. Overlays are never in reports or exports, so the finished drawing stands alone.
Files from other apps reach the overlay manager through the Share Sheet. See Bring files in.