The Precursors are as far as we know the first sapient species who then went on to seed the galaxy with life. They are presumably responsible for the plethora of different, yet similar life forms; especially considering the remote odds of life even occurring at all in space. Few, if any artifacts remain, and those that do are typically indistinguishable from the natural environment given that they’ve been there all along. The Precursors themselves however completely disappeared a long time ago. Popular theories include the idea that they ascended to a higher plane of existence or are simply waiting in dark space outside of our galaxy.
The Precursors did leave behind the Hard Light Sentients, but the facilities they typically inhabit are modeled after the Hard Light Sentient’s natural environment or a laboratory of their technology level with more advanced equipment disguised in plain sight.
No one actually knows for fact that the Precursors actually did exist, they are actually more of a myth or religion, and as such take on different aspects according to the different species’ viewpoints. Modern depictions of the Precursors are usually based on where the different myths overlap. The name “Precursors” is of course relative; no actual name for them exists because no one’s ever actually met one and evidence is scant to non-existent. They are just assumed to have existed to explain some of the mysteries of the galaxy.