Arduino Uno R3, Raspberry Pi 4 and 5 get four standoffs with pilot holes at the real mounting-hole positions. The Arduino Nano and the ESP32 DevKitC have no mounting holes, so you get four plain support posts under the corners. For any other board use the custom board: its size and four corner holes at a distance you choose.

No. Every opening is placed by you: choose the wall, the position along it and the height above the floor, and use a preset size (USB-C, USB-A, RJ45, HDMI and more) or your own. This keeps it working for any board and any layout. The panel shows the height of the board's top surface to help line things up.

By a friction lip: a ring on the lid that reaches 3 mm into the box with 0.2 mm clearance to the wall. If it is too tight or too loose, change the lip clearance and print again. For a firmer closure switch on Screw posts: four corner posts with pilot holes in the box and matching clearance holes in the lid.

Yes. The box and the lid are separate closed (watertight) meshes in millimetres. With the lid beside the box it lies flat, lip up, and the box prints open side up, so in most cases no supports are needed (the tops of wide port openings are short bridges). Export STL or 3MF.

Blender's default scene counts one unit as one metre, so a 74 mm box is 0.074 units. Set '1 Blender unit =' in the panel to 1 mm, or set the scene's Unit Scale to 0.001. The sizes in the panel are always millimetres.

Yes, as long as the box was made with Create Box and is still a closed mesh: select it, choose the wall and the opening and click Add Opening, as many times as you like. Boxes that you edited into an open mesh are refused with a message.