Figs seemingly don't need much soil to thrive!
It is the same tree from both sides.

Wow! This is amazing! Wonder how big this tree can grow before the wall is unable to support it's weight! Robin

Hard to say but there was one more tree near by photos of which I just uploaded. This one is quite big. I think from time to time the city takes care of this tree aggression.

Those kind are hard to root.   Better bring the whole wall with you.

What kind are they.? And were are you located. Must be California

Those are wild seedlings but edible. One is yellow outside and the other is green, both are strawberry inside. The figs are very sweet and small to medium sized.
The trees are in Switzerland .

I noticed fig trees growing in similar fashion all along the mediterranean cost starting from France to Crna-Gora. I bet there is dirt and soil behind that stone wall. And with the limestone and calcite deposit so common in europe the earth is porous and hold much moisture than other types. Notice in the pictures, there are mineral stains on the stone wall caused by trickling water/flow. Goodthing in Europe is they don't seal off the wall using cement & concrete like we do, unless they absolutely need to.