Yeah honestly. I know we (USA), do any number of things In a screwed up way, but it's also what I'm used to. There's NO border control anywhere around there? Seems weird to me.
When I was in Germany in 2022, I traveled freely around Germany, France, Belgium, Switzerland, and Czechia. I only got asked for my Passport in Switzerland, and then at the airports. Otherwise, no, you'd just be driving or on a train and the signs would change to French suddenly.
I loved it. I wish we fucking would relax our shit in the US. It was hella neat.
Do you have border control between states? It's basically the same thing with schengen. No control between countries on the "inside", but rigorous control to get in.
The lake is actually ~99% on Norwegian territory, it's just a tiny edge on the end that's given to Sweden and Finland. We once tried to give Finland a mountain peak though (for their 100th anniversary of independence from Russia), but the constitution states the kingdom is indivisible so the legal work was deemed too much. It would have been their highest peak if it went through.
If you slip, fall and hit your head and loose conciousness during that, in a way such that you are lying exactly on the border between two or three nations, to which hospital will you be brought? And how are insurances going to deal with this?
Seems like a non-issue to me. You'll go to whichever hospital is closest. If you're resident in one of the countries you'll be in EU/EEA and get the usual healthcare for residents of whichever country the hospital is in, if you're non-EU it'll depend on what travel insurance you have.