As a fellow Dutchy... my current place is the first place I ever lived in that came with a combination oven and microwave.

Previously I've always had to supply my own microwave / oven ;-)

I've seen it more there, for sure. Here I've seen it once or twice, in expensive homes that got a very early microwave when it was a luxuty and built it right into the furniture. Most other places have a secondary spot for a smaller microwave. I actually went and got a combination one, still smaller than many of the ones you see in the US, and found that it wouldn't fit in the microwave spot in my kitchen, so I ended up having it over a counter and using the microwave shelf as a spice rack.

Wait are these microwaves one unit, or a separate oven and microwave? Here in the states they're always separate, though sometimes a microwave might be above the stove and function as a smoke hood.

Something like this render from Best Buy is common enough. That's what I'd call a normal size microwave in either case though, just different mounting options.

I could certainly fit a microwave inside my oven, so they're not really too similar size here unless you get a moderately large one. In fact, that render only has a kind of weird scale, the gas range and oven seem about right.