A chev or ford can be serviced or repaired by anyone, anywhere with parts from any autoparts store.
but specific warranty work would require a dealer - if you are in an area remote from all dealers, then you are smart to consider that in advance- will they cover a 2 hour tow into the shop in the event that a warranty item renders the vehicle dead
for other services such as oil, filters etc etc look around locally.
there is no requirement that regular services be performed by a dealer to keep your warranty in force- in fact there is long standing Federal Regulation barring manufacturers from requiring that- unless they provide those services free-of-charge
BMW Mini division was in trouble for that last year.