Apple watch - taking it overseas to Europe and USA

I am looking at buying an apple watch 4 cellular which will be linked to my iPhone X. I have a regular sim and an electronic sim in my mobile, I know that the apple watch only has one sim.

I want to be able to use my apple watch WITH and WITHOUT having my iPhone X physically with me when I go abroad, eg skiing or hiking.

1 Can I use cellular on my apple watch in Europe without having my iPhone with me?

2 is there a difference between EE and Vodafone network when I go to Europe?

3 can I use my apple watch in the USA if I put a US sim into my mobile ?

thanks!