You use ‘magic’ as the handwavey thing in fantasy games. In a sci-fantasy game the handwave is much more likely to be psi or tech but for the truly outrageous stuff you want a handily extinct elder race that’s left all sorts of interesting ‘stuff’ around that doesn’t make any sense.
That’s your magic item equivalent.
Another trick is making blasters and lasers useful, but also making sure they don’t supersede melee combat because, well, everyone likes hitting things and who am I to deny them the pleasure?
That’ll be a tricky interaction between armour, force fields and weapons, but it shouldn’t be too complex as it comes out, as far as the players and GM are concerned. Other stuff you don’t need rules for so much, food pills, communicators and so on. A canndy GM can just handle that kind of stuff on the fly, no problem.
I don’t think I’ll have to go into too much detail, past the basics and that’ll leave plenty of room for other cool things.
Spaceships? Vehicles? These things are just big, big, big, scary monsters. Really, what’s the difference between a starfighter and a dragon?
Fuck all.
Postmortem Studios 
Yeahbut. When you use Precursors sooner or later all your stories will be about Precursors. Is that what you want?
Generally not a fan, but if you leave it utterly vague and have it more extinct species, than all-powerful alien gods, it can work.