Precursor Technology

"Any sufficiently advanced technology is indistinguishable from magic." -Arthur C. Clarke
The Precursors, being millions of years old, have had a lot of time to develop their technology. Essentially Clarketech, technology indistinguishable from magic. And oh boy, is it.
Let's start with where the precursors live. The ringworld. Now, while a ringworld is a real concept, first introduced or at least popularized by Larry Niven in his Ringworld novel, and has scientific research and theorization related to it, it is still not expected to be able to be built within a long time. The precursors, of course, did it anyway. A ringworld would have to be built from extremely durable and stable materials, of which there is no evidence of. Funny enough, not even the precursors had materials quite that strong, and instead they used artificial gravity generators to hold it steady. Yeah, they have those too.
Precursor nanomachines are really worthy of the title. Where human nanomachines are at minimum a few micrometers wide. Precursor ones are of course barely a nanometer. Don't ask how they manage heat, cryo-arithmetic engines or something. These machines can move in swars of trillions at a time, and form complex structures. The bodies used by the precursors are composed of these machines, allowing them to change shape rapidly and to suit whatever situation.
The main power generation in use among the precursors are black holes. That's right, those infinitely dense spacetime violators. They have succesfully miniaturized them for common use. Nearly everything from massive reactors on the ring, to spaceships, to the precursor bodies use these reactors. The black holes are obviously extremely small, and have barely any gravitational effect.
Precursors use bladed weapons at times. Axes, swords, whatever. Coincidentally, they use a system similar to a layered chain blade humans use, but with the chains of extremely thin teeth replaced with antimatter. Yeah. The particles are kept on the blade's edge... somehow, and the antimatter is generated in a small device at the base, manipulating quantum tunnelling for it's fuel. Yeah, precursors can manipulate fundamental forces for trivial things. Clarketech, everyone!
Ranged weapons are similarily ridiculous. A standard precursor pistol-equivalent can instantly vapourize a target to subatomic particles, or shoot a thin beam that removes, deletes, even, anything it touches. These weapons can be the size of a regular pistol, for standarf human hands, or mounted on their starships. A sustained barrage from a fleet could even delete a planet, if given enough time.
Starship propulsion, not FTL, I'll get to that later. The most powerful propulsion method in common use by humans is antimatter thrusters. They can accelerate a ship to, most often, around 0.7c at maximum. There have been experimental devices, like black hole propulsion, but that is way too experimental to see any widespread use. Once again, the precursors have beat humans to the punch, again. They use black holes, because of course they do. These generate gravitational waves, which push the ship along.
FTL, though, allows a ship to travel fast. Humans have cracked it, and use a form of Alcubierre warp drive. The precursors, as they do, use something more advanced. Teleportation, basically. The portals require a proportionate amount of time to charge relative to distance, but that aside there are not many limitations. They also posess instant communications, something humans have yet to discover, and consider impossible. Evidently not.
Another invention of the precursors is their real-time maps of the galaxy. Real-time. A full projection of all 400 billion stars, zoomable and manipulatable, allowing the precursors to monitor everything that goes on anywhere. They also have a collective record of everything ever recorded in humanity, and other alien civilizations, too. Every myth, every scientific paper, every political document, and every social media post. They do this to preserve records, in case something bad were to happen.
You want to get to the trippy stuff, dont' you? Well, let me introduce the Semi-singularity propelled strange matter-induced causality violator cannon, or colloquially known as an celestial eradicator cannon. Oh boy, here we go. This weapon is so advanced, so complex, that most precursors, save for the best experts, can't explain anything in detail about the weapon. But I shall try. The main principle of the weapon, the titular causality violation, is achieved somehow. I do not know enough to accurately describe anything even close to how it might function. This causes the target to both suffer direct damage, but the main attack is to shift the target backwards and forwards through time. This violates causality, a lot. Because the universe has a way of correcting causal violations, the target is often completely annihilated, or at least severely damaged. On the lowest setting, these weapons can destroy planets in one shot, and on the highest can even delete stars and even large parts of solar systems.

"Manipulating and violating causality, as both we and you humans do, is risky. While the universe has a way of repairing itself, often too severe a violation, or too long, can have catastrophic effects. Time travel, as you propose, would shatter causality so severely, even our greatest supercomputers, Matriyoshka brains, as you call them, can barely comprehend the effects."

— A precursor, upon being asked about time travel by Skejon.

TL:DR
The precursors utilise Clarketech, I use tech jargon.