He walks along the bottom of your screen, climbs up the edges of your windows, naps when you go idle, and on rare occasions wanders over to something he likes the look of and takes it. He always puts everything back. That is the whole bit.
Depending on which version you are running, "it" might be any of these. He gives all of them back. If he does not put something back, that is a bug and the author wants to hear about it.
He lives on a transparent, click-through overlay covering the whole virtual desktop, so he never intercepts a click meant for something else. He walks along the bottom of the desktop and bounces off the screen edges. Every few seconds he picks a random nearby window, walks over to it, climbs up the side, crosses the top, and climbs back down.
Minimized, cloaked, off-screen and shell windows get filtered out so he does not try to climb something that does not visually exist. After thirty seconds of you not touching the mouse he naps, and one twitch of the cursor wakes him up. He also detects fullscreen games and presentations and hides the overlay entirely, so he does not crash your movie night.
The process is per-monitor DPI v2 aware, so Windows cannot scale him out from under the renderer on a mixed-DPI setup.
creechr runs entirely on your machine. He sends no telemetry, contacts no server, and has no account or update check. The pixels he captures from other windows exist only in memory long enough to draw them somewhere else on your own screen, and are never written to disk or transmitted anywhere.
He is built from source and shipped as a work in progress, with the feature list updated each release. Go in expecting a desktop toy that is actively being built, not a finished product.