Själv is not only used for constructions with reflexive verbs, but also alone to mean by oneself or alone.

• Behöver du hjälp? (Do you need help?)

• Nej, jag kan själv. (No, I can manage.)

Or

• Vill du ha sällskap ikväll? (Would you like some company tonight?)

• Nej, jag vill vara själv. (No, I want to be by myself.)

Does that help?

"sig" is an object. Själv(a) emphasises the subject. English merges both of these concepts into all of the self-words.

For example

Han äter det själv. = He eats it himself. (Object is "det". "Själv" emphasises that it was him and no one else.)

Han äter sig (själv). = He eats himself. (Object is "sig" - indicating that the subject and the object are the same person. Själv is commonly used for emphasis.)

I see that you're learning German as well and it's very similar, so the German translations for these sentences are:

Er isst es selbst/selber. Er isst sich (selbst/selber).

själv(a) = selbst/selber sig = sich

I... suppose it could. But that it's sort of implied that it's a general statement with a formal subject. Anrui or Arnauti can decide whether or not to include it.