I see that humans can interbreed with either orcs or elves in addition to humans, and the offspring is either a half-elf or half-orc. I realized though, that this offspring is very much like a mule, a combination of two species: a horse and donkey. But the mule is sterile. Does this mean that half-elves cannot interbreed with other half-elves, and likewise for half-orcs? Or can they?
Half-elves and half-orcs are not sterile.
Xanathar's Guide to Everything, p62:
Nonhuman Parents. If your character is a half-elf, a half-orc, or a tiefling, you can use one of the tables below to determine the race of each of your parents. [...]
The tables presented below include "one parent was an elf and the other was a human," "one parent was an elf and the other was a half-elf," "one parent was a human and the other was a half-elf," and "both parents were half-elves" (among others). While this may vary by setting, the default laid out in XGtE indicates that half-elves can breed with elves, humans, and other half-elves.