Yes, it’s possible for a person to create software he doesn’t understand himself. In the old days this was pasting from Stack Overflow and changing things until it worked.

In the old days even if I knew how the software worked when I wrote it, I’d have no idea how it worked when I looked at it weeks later.

It’s also easy to modify software without knowing how it works. This produces modifications that hopefully appear to work, but that break other things, sometimes unknown things.