This info is in technical wire gauge tables. This is why the whole business is confusing. Cables are bundled insulated wires with little possibility for heat dissipation. In old houses with original wiring, "hot" and "neutral" 12 gauge wires are pulled separately from the main meeting only at receptacles etc. This makes them individual chassis with a much higher current rating.
Should the feed wire for 20 A kitchen circuits be at least 10 gauge?
What technical wire gauge table are you referring to? The only ones you need are in the NEC.
Yes, it can be confusing, which is why DIYers should not put such little importance on things like residential wiring. The whole "it's not rocket science" thing is BS. No, it's not rocket science, but it's not that simple either.
In some old houses knob & tube wiring was used, but not many.
No, #12 is breakerd at 20A. In fact, the actual amperage is much higher, but we are limited to 20A.
Why are you asking all this? What are you questioning and why?