Floor outlets (supply air) under sofa and desks?

Yep, you can see it from the slightly upward-standing hair and the facial expression

The PLUGGIT cover looks good, but somehow also like a shower channel.

Very stylish... Cat and Pluggit floor outlet^^ Physically, I do see the floor outlet as sensible. After all, with heat recovery, warm air comes out of the ventilation, and that rises upward. If the outlet is at the top of the wall, the fresh air is initially up there. Could you possibly take a picture with the cover grille removed? The background is that I have various manufacturing options (e.g., CNC milling machine) and am considering whether I could make a few personalized or simply more nicely designed covers myself... Can the outlets also be purchased without cover grilles?

Yes, the lower parts are simply molded parts made of sheet metal or PP.

Which grid is placed on top is usually left to personal preference.

A picture without a grid doesn't show much; it's just a sheet metal box.

Here are some variants:





And you can also have it round if you don't like the square ones.

So of the ones shown here, I personally only like the version that is flush with the floor. All the others would bother me; at least if they are in a conspicuous place.

They are all flush with the floor.