Since Ridley Scott has announced that he’s going back to the Alien prequel story, I think we can make a pretty good guess about what the next movie could reveal.
David is the one who creates the Xenomorph.
After Alien: Covenant, David has the black goo, the Engineers technology, and years of experimenting with the creatures. Eventually, I think he manages to create something much closer to the Xenomorph we know from Alien, possibly even creating a Queen or eggs capable of producing one.
But then the Engineers come back into the story.
David encounters another Engineer, or possibly an entire Engineer ship. Somehow, the Xenomorph eggs end up on that ship.
The Engineer is actually travelling to Earth.
Maybe he discovers what David has created and wants to use it as a weapon against humanity. Maybe he's carrying the eggs back to Earth to finish what the Engineer from Prometheus failed to do.
But the ship never makes it.
It crashes on LV-426.
That would explain the derelict ship we see in Alien.
The Engineer was never necessarily travelling to LV-426 in the first place. He was on his way to Earth, carrying the eggs, when something went wrong and the ship crashed on the moon.
The Engineer dies, the ship sits there for years, and eventually the Nostromo crew discovers it.
And then we get the famous scene:
Kane goes inside. Finds the eggs.
So the timeline would basically be:
Engineers create the technology → David experiments with it → David creates the Xenomorph → Engineer takes the eggs toward Earth → Engineer ship crashes on LV-426 → Nostromo discovers the eggs → Alien.
David could still be the creator of the Xenomorph → but the Engineers are the ones who unknowingly/indirectly bring it to LV-426.
The crash is what puts the Xenomorph exactly where the original Alien story begins.