Basically there are two ways to produce seedless fruit. One is to have a "sport" tree that, while it came from a seed, produces seedless fruit. You then use vegetative propagation through cuttings or grafting on other trees of the same species to produce more trees. This is how naval oranges and Clementines are produced--although on occasion the fruit reverts. The other is through breeding. Some crosses produce plants that have sterile fruit--this is the way seedless melons are produced. Usually the flowers that develop into the fruit are lacking parts that prohibit complete fertilization. The plant is fooled into thinking it is producing protection for seeds that do not develop. Often the fruit does have "seeds"--you will often find immature seeds in seedless melons. It is also possible to produce seedless fruit by use of chemical inhibitors--kind of plant birth control--but. last I knew, it was too expensive to do commercially.
BTW, I messed around growing tomatoes in my greenhouse but, because the flowers didn't get pollinated, the resulting fruit was seedless. They were also very small. The big commercial greenhouses actually have resident bees to take care of this.
