Microwave ventilation holes are shaped like electric plugs?

That’s because those aren’t ventilation holds, they’re holes for plugs. They must use the same backplane worldwide and use different electrical connections in different countries. Also, the same backplane may be use for different models. Why retool when you can make one generic backplane and reuse.

The 3 rectangular holes look like they’d fit UK/IRL plugs. The two circular ones on either side of the rounded rectangular (or oval?) shaped hole look like std. EU plugs.

….maybe one reason is that it comes out looking like a funny smiley face.

the company who makes your microwave uses the holes as they manufacture. They plug the cord in to it to help keep the cord out of the way, and not to dangle