The Stargate Universe is fairly easy to explain.
A long time ago in a galaxy far, far away...
It is a period of civil war. Rebels, tired of being hunted while hiding, have decided to run from the evil Galactic Empire.
During the departure, rebel scientists managed to dream up secret plans to the ultimate weapon, the Ark, an armored space box with enough power to destroy an entire planet.
No Longer Pursued by the Empire's sinister agents, the rebels land in a new galaxy, hoping to save their people and restore freedom from overbearing religious nuts...
The Alterans leave the Ori, and settle in the Milky Way. At which point they start seeding life into the galaxy. Including humans. They also start expanding, creating various city/worlds, expanding their technological pursuits, planting Star Gates everywhere. Then the plague came. They were quickly dying, even time travel didn't help (it never does). So the last few of them packed up again, and left to the Pegasus Galaxy. In the process, tons of Atlantian technology is left everywhere. I'll come back to this.
After once again running away from a fight, the Ancients return the the Milky Way, very few in number. Some bred with cavemen on earth. Some found vacation homes on various Milky Way planets. Some stook around and created the Alliance of Four Races, but that only lasted a few hundred years. Some Ascended. In this time, Asgardians were trying to understand Ancient Technology through the Repositories. Different groups of advanced humans have sprung up in isolated pockets, descendants of Ancients or using Ancient Tech.
After the Ancients have faded into myth, and the Alliance pretty much crumbled, the Goa'uld came into power. And how? By doing what everyone else seems to be doing. Finding lost Ancient technology. The use of Star Gates allow for instant transport to other places the Ancients might have left more tech. And the Goa'uld for one flourished because of it. First by Stargate, then by Ancient Ships. Most if not all of their technology is based on Ancient Tech. They find earth, and begin to use humans as livestock, spreading them out to other planets as slaves for naquadah which fuel stargate technology, or genetic engineering them into Jaffa with genetic manipulation devices the Ancients used to become the most advance short of ascending, or Sarcaphagous based on Ancient Zombie, er Healing Devices. And even human technology (drones/recon devices) or anything else they could reverse engineer.
And many slave planets, full of Ancient Gene possessing earth based humans rebelled against their Goa'uld, and started doing the same. Using Goa'uld and Ancient tech, blending it with their own. In essence,the proliferation of the Ancient Technology, open access to Star Gate travel, and humanity's diaspora, stunted creativity and long drawn out individualistic technological creation for a Scavangers dream. Treasure trove of easily reverse engineered technology with a heavy basis on Energy Weapons. Pretty much the majority of aliens we see in the Stargate Universe are using Ancient Technology, so there is little variation.