This isn't a European standard, I believe it originally came from Germany and a lot of other countries followed suite. In these countries the electrical installation boxes are usually circular with a 60mm spacing between mounting screws [1]. Compared to the UK which uses square boxes and US which has rectangular boxes, this is probably because walls in Europe are usually made from masonry (and often concrete) and it's much easier to drill a round hole than a square hole. The boxes themselves are plastic. If you want multiple you can just place multiple single units next to each other, or you can get installation boxes that are multiple units too.
I believe one of the first companies to come out with the idea of mounting them in multiple 'units' was Gira with their "System 55" line. [2] The switch / socket / dimmer / whatever, called an insert, goes into the installation box and usually screws in with at least two screws. The part that is exposed is usually 55mm (hence the name), and a frame fits snugly over the top, covering the screws. It's worth noting that this isn't a standard, and they are not always interchangable between manufacturers - for example the spacing between frame may be slightly different. In my apartment for some reason the electrician decided to use a different manufacturer for some switches, and they often catch on the frame and aren't as smooth to operate. The frame just snaps in place over the top, so often it's tricky to get all the inserts aligned properly for it to fit correctly.
When you buy them from a hardware store you buy the insert and the face plate seperately. There's usually a few different colours to choose from, in my country the hardware stores sell white, ivory and anthracite, but if you go to a specialist electrical retailer they will have more to choose from. For example Busch-Jaeger has over 10 different designs to choose from [3]. Some manufacturers even sell the switch plates - the bit you touch - seperately from the insert. This makes more sense if you think about commercial installations, where you may have different printed symbols on the plates, but behind it's all the same. It's also worth mentioning that the inserts are not just limited to switches or sockets, you can get heating controllers, radios, motion sensors, smart home controllers, etc [4].
[1] https://www.lichted.de/led/montageanleitung-zum-einbau-von-u...
[2] https://www.gira.com/en/en/inspirations/systems/gira-system-...
[3] https://www.busch-jaeger.de/en/products/switch-designs
[4] https://www.jung.de/en/623/products/technology/