Seriously ever since they introduced it they've never explained wth it actually is. How are all these characters who don't have it able to compete with people who do?
God Ki is on another plane compared to regular ki. Like going from unleaded gas to premium or whatever.
Goku and Vegeta have the foundation of what it takes to become as powerful as a god of destruction. But they are still in the infantile stages of unlocking its full potential.
Think of it as during the very beginning of Dragonball, Goku learned how to manipulate his ki but could still get his ass kicked by some of the strongest humans who had no understanding of ki or how it works.
That's where we are at in Super right now. Goku and Vegeta have been exposed to this new form of ki but the strongest individuals in the universe can still give them work because Goku and Vegeta are still learning how to utilize this new ki.
The difference is the ceiling for Goku and Vegeta has just been raised 10x and maybe even more.
Look back at all the times Whis, the Kai's, Beerus, etc talking about the forms or the ki itself. They explain it but I admit it is done very poorly.
Skip to 1:18 when Goku transforms into SSB. You can see the look on the Kai's and the Gods of Destruction's faces that this is something they've never seen before, at least not from a mortal. They even point out the unique "pressure" that god ki puts out.
shyts not complicated it just poorly explained in the series.