most airlines are part of an alliance and use code sharing.
For example air Canada has some the best award redemptions and is part of star alliance. So I use it to book Singapore to Bkk. Then I use my American miles for the return ticket on One World alliance.
no. They are not. It is much more intricate and complicated along with repercussions and consequences. So either you’re not a US citizen or a nincompoop. Maybe a combination of both.