1. Has somebody experience with them?
2. Can somebody say something about the "quality of their engineering"? I am currently getting into microelectronics and wanna learn by examples, but obviously I can't say whether something is the equivalent of clean or spaghetti code

They're mostly using off the shelf chinese esp modules. They look like very polished hobby projects.

I'm a little envious of someone making what they want and selling it to people who want to buy it.

I have a macropad8, it’s nothing special, it works, the provided “case” is somewhat lacking (just two sheets of lasered acrylic with spacers), but there are 3d models for better ones online.
I appreciate the fact that everything they do is released as open hardware though!

Today I learned I learned...

You mean the Seeed XIAO boards right? Because most what I saw of them they were using the RP2040 chip (which is not ESP under the hood...is it?)

They look like very polished hobby projects.

So good learning examples?

RP2040 is another jellybean part, much of a muchness. I've got both on devboards in my parts box.

a good learning example would be recreating his designs yourself in CAD and writing your own firmware, or rolling your own version of QMK or whatever its running