How do they reproduce seedless fruit?

either they have seeds and theyre just to small or they cut off and clone parts of the plant to make more

Well, for many seedless fruits that grow on trees (oranges and their relatives), all species are grown using a hardy rootstock (often a species of sour orange which produces unpalatable fruit, but has really great nutrient uptake and water holding capacity), and bud cuttings from the desired fruit are grafted to the tree. Then all the rootstock leaves are cut off, making the seedless fruit the main trunk. It is best to clonally propagate and graft seedless and seeded trees, because allowing willy nilly pollen to create seeds that are blends of multiple species will not create tasty fruit.

Seedless watermelon/banana and other herbaceous fruits are from plants where either fertilization is not allowed to happen and a fruit with no mature seeds forms asexually OR the plant itself was bred to be triploid (3 sets of genes, as opposed to normal diploid, or 2 sets of genes). This means that meiosis, which forms seeds, cannot take place normally and you have seedless fruit! But triploid seeds themselves are viable, so you can plant them normally.