I am not certain how it works in all of Europe, but it doesn't seem possible in Sweden or UK.

In US and Japan, they will rather do the full roster of tests than transfer in radiology results from another hospital. Its not something to do with costs, but physician opinions. Every specialist has his personalized view of the patient in hand - so the corpus of information he needs is different.

As far as I know, in Sweden data about a patient is shared between every hospital... you need to sign up on a web portal, of course, but from then on, you can authorize a doctor or nurse anywhere to get your data with a click... you can go to any clinic and they have access to your full history. It's impossible to do anything properly if you don't have that. I just don't understand why in the US, it's acceptable for hospitals to be silo-ed like that to the detriment of everyone's health.

Here's a news report from 2017, when the system was being set up: https://www.bmj.com/content/357/bmj.j2069

Today, this has been working for years.

The US has multiple individual medical systems larger than Sweden with essentially the same abilities. Sweden has roughly the same number of hospitals as South Carolina.

What makes you think the Swedish system wouldn't scale to countries with 100's of millions of people??

Serious question - could it? How would that be accomplished?