The airline you fly & the airline ffp you credit those flights to does not need to be same.

Freq flyer miles are not equal to earn or burn.

USA based airline ffp's tend to be more generous (status benefits/earn/burn/upgrades/award cost/award cash surcharges/expiry/affiliated credit cards/promotions) compared to non USA based airline ffp’s (even after the recently announced changes to some USA ffp’s)