Yes it has full world coverage, see the esa navipedia for more info.

No idea how it is in the US, but in EU you can't really easily choose, it will by default use GPS+Galileo combined for better accuracy.

I guess you can through developer settings on android though.

If you're on Android there's an app, GPS Test that shows you a chart with all the signals it's picking up with different symbols for each system and a filter too. My phone is currently picking up 5 GPS, 4 GLONASS, 8 Galileo, and even 3 Beidou sat signals (I'm in Western Europe).

Also you should be able to look up your phone specifications to see what GNSS services it's capable of using on GSMArena

Edit:oh I missed that you're on Apple so that app is useless for you. Still might be of some use to others though.

well i do not share your "enthusiasm" do not forget those systems used twitter,google,amazon,microsoft infrastructure, so those private companies build it first...