The ivy in this sycamore did not just climb up from the ground to hang over this big limb. If you look closely, you will see no vine coming up from the ground. This small patch of ivy as well as a small pyracantha, are growing in a decayed cavity on top of the big limb. The ivy may have climbed up a long time ago, and then rooted into the cavity before the original vine was somehow removed. Alternatively, the ivy might have grown from a seed that was dropped by a bird or ivy vines that are higher up in nearby box elder trees. It is impossible to say now.It is also difficult to say why there is such a large cavity on top of the limb. It could have originated as a large scar incurred from the impact of another large limb that fell from above. There are a few cavities higher up that were caused by large limbs breaking away. Although unlikely, the cavity could have developed from sun scald damage, after the upper surface of the big limb suddenly became exposed by the loss of limbs higher up.This other aerial patch of ivy hangs from a smaller cavity higher in the same tree. Oddly, among sycamores, such cavities on upper surfaces of large limbs are not uncommon. Sycamores often drop large limbs from high in their canopies, exposing or damaging lower limbs below. Their lightly colored bark is very susceptible to sun scald if suddenly exposed after always being shaded. It is also not very resilient to heavy impact. Of course, more typical cavities develop on trunks where significant limbs broke or were pruned away. Other plants can grow in these as well, as demonstrated by the ‘epiphyte’. https://tonytomeo.wordpress.com/2017/11/15/epiphyte/