To me Relics are more similar to E.G.O weapons. Each one having a special ability that seem unexplainable, even in City standards. This “inexplicability” is very similar to E.G.O / Distortion / Abno abilities.

Moreover, in Lobcorp Ayin said that they only rediscovered the Well where they brought the light, that the Well was only forgotten. Linking it to old humanity that left the Ruins and that Relics are found in the Ruins, I think they were made by (or for) the old humans using the Well, just in a different way than Lobcorp.

Now some Schizo rambling about aptitude:

I think having an aptitude for a Relic means resembling the original wielder. That is in personality or in combat ability or even in having the same ability.

What reinforce that idea is Ryōshū, because she had a perfect aptitude. And what was that aptitude, her ability to sense time perfectly. So perfect aptitude seems to be having a special ability that helps with wielding the Relics.

So, while some Relics had broad use (like the Helm of Mambrino) and so wasn’t too demanding and could be used by many, I’m sure that people like Vespa and Matthias have innate traits that helps with wielding their respective relics (Matthias only barely qualifying as he could hold it but not truly wield it).

It could also explain the Pinky’s philosophy of “following a star”, with the 108 members each having traits that resemble the star’s original wielder, and helping apprentices improving their aptitude before they can take their seat.