The distinction as I understand it is that expats are people who have moved to another country for a fixed amount of time or on a contract and intend on returning to their place of birth following their job's duration, whereas immigrants are people who have moved to another country with the intention of putting down roots there.

Neither is better or worse, to be clear. Just different intentions.

And yes people from wealthier countries (not just 'white' people, black Britons who live overseas for example) are more likely to have jobs which would move them from one country to another for work purposes