(SPOILERS) Origins of Xeno & Aliens? Help & Answers Needed

Spoilers included so if you didn't see the new covenant film or wanted to please stop reading!

I am looking for answers regarding the aliens and the origins of the aliens in the films. I always thought the aliens (black carapace and funky second jaw creatures) were like an alpha predator on a world or planet then taken and used as a weapon by the "engineers" in the Promethius/covenant movie. The novel by Alan Dean Foster states that the space jockey race found them on LV-4whatever, and there wasn't really any conclusive evidence shown (up maybe till now) that they were created? 

After the new film I am not sure. David the AI psycho robot mentions he engineered them? Does that mean he did so from scratch - or did he alter an existing form that already existed? 

Or am I totally wrong in thinking they existed as a species? I would like to know their origins and if they were created after they mutated from contact with the black goo or were engineered from scratch?

Any mature posts are highly appreciated and thanks!