In the Stargate franchise, the basic idea is that many aliens have assumed the roles of Terran mythological figures, including the Goa'uld (Egyptian), the Asgard (Norse) and Alterrans (Buddhist?). My question is, did they create these religions or did they just use them to gain control over humans?
The Goa'uld didn't create the religion, they high-jacked it.
From series 1, episode 1 - Children of Gods
HAMMOND Then who's coming through the Stargate?
DANIEL Not as in "God" god. Ra played a god, the sun god. He borrowed the religion and culture of the ancient Egyptians he brought through the 'gate and then he used it to enslave them. You see, he wanted the people of Abydos to believe he was the only one.
Here Daniel is saying that Ra borrowed the religion.
According to Wikipedia Norse mythology developed due to the Asgard coming to Earth.
In the series, the Asgard gave rise to Norse mythology on Earth, as well as accounts of the Roswell "Greys".
And from Stargate wikia
The Asgard were a benevolent, extremely advanced race from the Ida galaxy that visited Earth on many occasions, giving rise to Norse mythology.
The Ancients don't pose as Gods, they have strict rules surrounding not interfering with humans and other non-ascended life forms.
From series 9, episode 2 - Avalon: part 2
VALA (interrupting) Excuse me. Daniel, what if these gods, the Ori, are the people left behind by the Alterans?
DANIEL You think they ascended?
VALA It's possible. The Alterans left a long, long time ago, and what we know of the Ancients, they learned to evolve and ascend. What if the people who remained here did too?
DANIEL That would make these people a subsequent evolution of humans, which is apparently what happened in our galaxy after the Ancients we know ascended.
VALA And it would explain why they're not as advanced as we might expect.
DANIEL No, but the religion doesn't fit the profile. The ascended beings I know don't pose as gods. I mean, that's the one explicit rule they DO follow, is that they don't meddle in the affairs of the lower planes of existence.
The relation to Buddhism appears to be coincidental. The only link I can find is that Oma Desala lived in a Buddhist-style temple. There's nothing to suggest this is connected with Buddhism on Earth so this seems to be just a resemblance rather than a connection.
The Ori use religion and peoples' worship to gain power. It's not clear whether they created the religion to suit their purpose, or whether their presence inspired a religion.
From series 9, episode 3 - Origina part 3
DANIEL I guess what I'm trying to understand is whether the Ori have spoken to you directly and told you to worship them, or whether you've…misinterpreted some evidence you've found along the way and developed this religion on your own.
tl;dr They must have been playing out characters from an existing mythos.
The Goa'uld characters don't just represent a single, point-in-time pantheon character list. There's a whole family heirarchy of wives, husbands, sons and daughters, which develops over time, and the way the Goa'uld play these roles almost entirely mirrors those of their historical, mythical namesakes (as referenced by Daniel every time he goes into one of his books).
This was never terribly believable for me, because you'd basically need to engineer your entire society's history as theatre, which is silly and does not mesh with the Goa'uld's preference for self-gratification.
But the alternative, that the myths came from the Goa'uld rather than the other way around, does not really fit the timeline; from what I remember, many of the references Daniel digs up predated the respective Goa'uld activity by hundreds or even thousands of years.
I always thought it was inspired as the film says the Goa'uld came to Earth ten thousand years ago, before Egyptian Culture really existed and Ganos Lal implies that when the Lanteans (Ancients/Alterrans) returned to Earth via the Egyptian Gate.
When we first abandoned Atlantis all those years ago the Earth was so harsh, it’s people so primitive by comparison that there was no hope of living amongst them as Lanteans and rebuilding our society, so we spread out, to many lands. Some planting the small seeds of civilisation among the first tribes of man, some made their way to the Stargate at your Southern Pole. Still others chose to spend the rest of their lives in seclusion and meditation…
Which is highly suggestive that the beliefs of native were influenced at their inception by the Goa'ulds