It is less "sim card is tied to this phone and can't be moved" and more "sim card exists logically and doesn't require a physical presence in the phone it wants to be used in"
Which really sucks for travellers. High roaming costs? Just go to a gasstation and buy a travel sim card, put it inside, use it, put it in the walled when you leave, and if you still have any data left, use it the next time you come there.
Just go to a carrier website, buy an esim, download to the phone, use it, switch it off when you leave, use it next time you come there.
It is literally the same except you don't even need to go to a gas station. My Pixel can hold as many esims as I want standing by until they are needed to be used.
