In a campaign you get the jet assigned to the squadron you choose
When you first go into a new campaign select a squadron from the map - and the model of jet will change
In the Balkans you will probably have a choice of F-16CG (block 40/42), F-16CJ (Block 50/52) and F-16AM (MLU) from what I remember
If you create a TE or fly in dogfight mode you can fly the other F-16s also