Side question - always been curious. Surely these houses were not designed for plumbing, electrical, efficient insulation, etc. Do they just bodge these things onto the outside of walls, or how does that work?

I've lived in a couple of houses built in the mid 1700s with parts even older than that. They were cob cottages, one with a thatched roof, so very thick walls instead of insulation. Plumbing and electrics were just added over the years. Windows were single glazed, so not very efficient. But pretty comfortable nevertheless, provided you remember to duck when going under beams. Houses this old are not unusual where I live in Devon, but most are more recent.

https://en.wikipedia.org/wiki/Cob_(material)

Many of them are basically not insulated; the vast majority of "old Europe" is in more temperate climates for obvious reasons. And in the colder areas, you have the immense thermal mass of brick/stone buildings. Sealing air gaps provides some help.

Plumbing and electrical is "simpler" to run than it may seem, but it does result in unexpected locations for bathrooms, etc, as those were often added wherever they could fit them in.

Remember that given the population growth, the vast majority of the people living in Europe are not living in 300+ year old buildings.

Mixed, but sometimes there's trunking on walls yes. I have a little bit of that (for something disused actually, plan to remove, I've not long moved in) but it's mostly under floorboards. (There are also would-be exposed pipes in the kitchen, hidden behind counters, I believe - but they probably would have done that anywhere, that's a relatively modern extension.)

Insulation is a more complicated topic - if you're going to do it you need to do it differently, since modern new-builds and modern insulation is designed around making everything air & water tight, which will make an old house very damp and rotten (which will lead to woodworm/boring beetle) - the structure needs to 'breath'.

Indeed. There are many houses in Europe that survived 3-4 different states claiming the land on which they stand.

Well, I live in England, so unless we count devolution (of some powers in other UK constituents) or changes of royal household, just the one in my case ;)

England is pretty stable. Some places on the continent, not so much, esprcially to the east and south of modern Germany.