They’re theorized to be fundamental particles. Nothing smaller exists within them. There’s no nuclear decay of or fusion which produces electrons. Similar to how photons are quanta of energy.
Edit: clarified that electrons don’t decay. Of course things decay into electrons plus other things.
Er ... there are lots of nuclear decays which produce beta particles electrons. Neutrons have a short half-life when free of a nucleus, and decay into a proton, electron and an electron anti-neutrino (to conserve momentum). [1]
Electrons are posited to be fundamental, and e-e e-e<sup>+</sup> collisions haven't, as far as I know though well outside my original field of study, produced any data suggesting internal structure.
This doesn't mean that they don't have structure, we simply have no theory (that I know of, but then again, I'm a former solid state guy) that predicts structure, nor do we have sufficiently powerful colliders to get us to a point to see such structure.
