OK, so I have the 2009 edition of the Marvel Encyclopedia, which has 8 pages devoted to The Hulk. Apparently he had “every atom of his body exposed to gamma radiation” from a gamma-bomb he was working on. It’s been revealed that Banner also has multiple personality disorder that contributes to his various Hulk forms.

I thought I remember from decades ago that his bulk and healing energy comes and goes from a transdimensional space, but I can’t seem to find that in the article.

However, I really like this StackExchange answer, which posits that he is an energy-processing mutant that taps into the Power Cosmic to fuel his incredible strength. https://scifi.stackexchange.com/a/84410

Wait until you see the more recent lore that explains that Gamma radiation comes directly from super hell and there’s magic involved. It technically doesn’t make what is in the encyclopedia untrue, but it wildly recontextualizes it.