I have a question

I sent 10 bombers and 8 tanks in two air transports (exactly in this sequence)
and then my eight tanks were consumed first
destroyed all enemy units and left eight bombers and the two transports, however couldn't get the city because there was no land unit.

wanted to know what the order of attack by units
(actually does not seem to follow any)

Yes, units with highest attack - attack first. Units with highest defense - defend first.

Your movement order (if done in the same turn) won't interfer in the order that the battles happen. It will follow the order already explained by Caulerpa.