I’ll add a little to wittman’s response above if I may.
As he says all units defend, so if a tank, fighter (you call it an interceptor?) and bomber are in a territory that is attacked by land and/or air then all the units in that territory participate in the resulting combat. This is regardless of whether you are attacking with land forces only, air forces only, or a combined land and air force.
Fighters can be used as interceptors in the case of a Strategic Bombing Raid (SBR). An SBR is an attack on a factory, air base or naval base, not on the territory nor units within it. However the defender can decide to launch his fighters to intercept the attacking air force in the event of an SBR, in which case these defending intercepting fighters (and only these) take part in combat.
Turning to the surrender question. Again merely to add to wittman’s reply. The concept of defenders surrendering does not exist in the game. The defenders’ only option is to keep fighting for as long as they are being attacked - i.e. until they are wiped out, until the attackers are wiped out, or the attacker retreats. Attackers may retreat at the end of any attack and defence full combat roll.