How do navel oranges work? I mean if they don’t have seeds…

According to wikipedia:A single mutation in 1820 in an orchard of sweet oranges planted at a monastery in Brazil yielded the navel orange, also known as the Washington, Riverside, or Bahie navel. The mutation causes navel oranges to develop a second orange at the base of the original fruit, opposite the stem. The second orange develops as a conjoined twin in a set of smaller segments embedded within the peel of the larger orange. From the outside, the smaller, and undeveloped twin leaves a formation at the bottom of the fruit that looks similar to the human navel.

Because the mutation left the fruit seedless and, therefore, sterile, the only means available to cultivate more of this new variety is to graft cuttings onto other varieties of citrus tree. Two such cuttings of the original tree were transplanted to Riverside, California in 1870, which eventually led to worldwide popularity.

Today, navel oranges continue to be produced via cutting and grafting. This does not allow for the usual selective breeding methodologies, and so not only do the navel oranges of today have exactly the same genetic makeup as the original tree, but also, they all can be considered to be the fruit of that single, over a century-old tree.