ttrpg

First off, do NOT just take stuff from High-Tech or Ultra Tech and plug it into your game. The numbers will not work with a low-tech fantasy game , and would have to be drastically adjusted, though you can of course use them for ideas.

Since we're back on designing your own stuff again, the first thing is to decide what rules you actually need for your players to interact with the ancient tech satisfactorily. Imo this comes down to how to use/repair ancient tech, and stats for robots. Equipment can be built as powers the same way you would build any other magic items, though if you're really dedicated you could work out an entire separate system for the technology/magic to really make them feel distinct. If so, Thaumatology is generally the book to look to, as it contains a bunch of magic systems, though they will all take some work to implement fully into a campaign. A lot of GURPS stuff will suggest using Quick Gadgeteer for high-tech gadgets but imo that's a really hand-wavey system that's not very interesting.

The main thing is going to be getting the numbers right. Damage and armor for laser guns or robots just need to be pretty close to what players can do. For damage, a single-target laser beam-type attack should cap out around 6d damage (which is a lot), area effects like flamethrowers should cap out around 2-3d, and armor should never get that much higher than the heaviest plate armor which IIRC is around DR12. Robots probably have Damage Tolerance: Unliving, since they don't have vital organs (unless you want them to), and Electrical or even Vulnerability (Electricity) to represent a susceptibility to lightning magic. As for pieces of ancient aliens tech, maybe they need ammunition in the form of ancient battery packs, or are vulnerable to electricity, or have a malfunction rating (which might be higher if used by someone who doesn't know what they're doing) where when attempting to use them they can "jam" (or explode!) and need to be fixed before they can be used again.

In terms of PCs interacting with ancient stuff, maybe add a Hidden Lore (Ancient Civilizations) skill to understand the writings and tools of the ancients (eg knowing what an "On/Off" switch looks like), and Engineer (Ancient Technology) to use/repair ancient tech. A full Language (Ancient Civilizations) is also possible if you want them to be able to talk to robots without the robots having to know whatever the setting's default language is.