Do you mean aside from the fact that that’s what he does? Like, do you mean the actual mechanics of it?

No.

He was not based upon anything in particular.

I don't remember our discussions clearly (it's been over a decade) but I think Liam is the DNA donor.

He’s as strong as both of them combined and can grow as big as Humungousaur.

If they didn’t do it in the show, it isn’t canon.

But maybe if we continued our show, we might have shown Swampfire and Wildvine getting energy boosts from sunshine. Maybe.

How hot is NRG for him to require a containment suit to protect the environment around him, and what type of radiation does his body emit?

His containment suit is to protect the environment FROM HIM, not him from the environment. Right?
If he were in space and took off the suit, he might go supernova like a small star and not be able to be contained again. Maybe. I’m just riffing here.

You mentioned that NRG could survive the Sun or volcanoes as long as he is wearing his armor. But would this also apply to the vacuum of space, or would he not survive even with the armor? Or is this protection specifically for extreme heat only?

If Duncan said something about the original series, agree with him. I wasn’t there, plus he & the rest of the MOA created those characters.

Mrs. Heatblast shown in Ben 10: Original Series and in Ben 10: Omniverse the capability of absorb fire and heat (A lot of merchandises confirms it, and Duncan said that he can even absorb blue fire).

Could NRG and/or Swampfire do the same?

And this changes something about absorbing solar energy?

Heatblast could survive contact with volcanic lava but not for long, because he would quickly become one with it.

Their heat is internally generating, not externally; so they don’t get their power from the sun or volcanoes. Certainly not NRG who wears a containment suit, he is so hot.
I think that NRG could survive the sun or volcanoes as long as he is WEARING HIS SUIT.

Echo Echo when he is multiplied

The solar plasma “from” the sun continues to exist in space “away from” the sun, not just “on” the sun.

(I’m trying to choose my words so that Google will translate them for you correctly! 🙂 )

They lay their eggs in the extreme coldness of space, not “ON the sun.”

Necrofriggians, even as cold as they are, must be intangible when “ON the sun” in order to survive its extreme heat. At a safe distance from the sun, they can be tangible and feed on solar plasma.

To summarize, the sun is too hot even for Necrofriggians to be “on”! 🙂