It’s the same as with all military technology: everyone develops their own, partly because of differing demands and partly to keep the capabilities secret from (potential or actual) enemies. The Germans had the most extensive bunker design and construction program, followed by the French, but most other European countries before and even during the war built bunkers of various designs as well.
No, the Germans built their own designs but not those of other countries. They did, though, take over fortifications in occupied countries, of course — often enhancing them with more German bunkers if needed. A good example is the
Kornwerderzand complex, which was constructed in the 1930s to defend the eastern end of the
Afsluitdijk, as that was essentially the back door into
North Holland. This is the only place in Europe where the German advance in May 1940 was stopped, and apparently, the defenders were quite surprised when they were told to lay down their weapons because they didn‘t know that most of the rest of the country had been overrun. Anyway, all the Germans did after taking over was construct three additional bunkers in 1943, of standard German design, but seem to have considered the defences perfectly adequate otherwise.
It was a headache anyway
Most German bunkers were constructed by a combination of local workmen and forced labour, controlled by the Organization Todt, and used little machinery but a lot of manpower. Because German industry couldn’t keep up with the demand for things like armoured plates, doors and turrets, eventually the Germans ended up stripping these from the Westwall (the line of defences built in the 1930s in southwestern Germany, opposite the Maginot Line) and even then, you got things like:
View attachment 451272
This is where an armoured door
should have gone, but it appears none was available before October 1944 — when reaching this particular bunker with building materials became impossible — so it remains as rebar sticking out of a concrete wall almost 80 years on. (Well, I took that photo about 20 years ago, and I haven’t visited this bunker in years despite it being under ten minutes’ walk away, but I have no doubt it’s still like that now. )