who makes this stuff?

[Alice_89th]

If I remember correctly, the SMN and SCH relics are brought up by people as stuff of legends.
Because both were more or less lost arts, there aren't many people that know anything about them anyway.

I can't remember what kind of book the other SMN in the job questline wields. Same with the SCH questline, not even sure we saw a book in that one.
EDIT: the SMN appears to be using Ifrits Grimoire. The SCH (in the small glimpse you get) does have the Omnilex equipped.

[Falen]

Lazy speculation is lazy; it's entirely possible the people we got the Jobs from had their own "Relic Quest" that each of them (like Jehantel, Solkzagyl, and others) completed. For all we know, there could be a legend tapestry (think Wind Waker opening) in the Sultansworn areas that describes or shows the Curtana and that inspired Solkzagyl to it where others wouldn't have cared. Or, only as a free paladin was he able to go searching for the clues to it.

[Triston]

In the SCH questline, the Omnilex was widely used as a combat manual by ALL the Scholars of the time; it's not unique in the sense of the other artifact weapons. When you acquire it it's even specifically acknowledged as "a copy of the Omnilex."
This doesn't explain the multiple Artemis Bows, Curtanas, etc. but at least for Scholar it's well-established that every Scholar worth his salt had a copy of the text.

[Shaezerus]

Lazy speculation is lazy; it's entirely possible the people we got the Jobs from had their own "Relic Quest" that each of them (like Jehantel, Solkzagyl, and others) completed.

Strangely enough, Widargelt is unique in wielding an unfinished relic weapon; his Sphairai are noticeably duller than your own. I'd thought Estinien's Gae Bolg was using the unfinished model as well, but checking the cutscenes again this isn't the case, it's just poor lighting.

[monochromicorn]

In the case of the WHM storyline, it can easily be explained by the fact we are visited by the spirit of the original wielder of the Thyrus, A-Towa-Cant appearing. So it makes sense that he'd have a spiritual representation of his weapon. I have not done the other quest lines for PLD or BRD so I could not say about those appearances. Ququruka in the BLM quest also uses a Stardust Rod too. I'd have to look closer into the SMN and SCH quests, a book is a book unless you pay close attention.

you are right about WHM relic, but the BLM is also explained easily. while speaking to gerolt, he says that there was a mage who made them, and that he had seen one himself, that was later stolen from the auction by the kobolds. there was more than one stardust rod made.

[Empressia]

Pretty sure Oathkeeper is made of mythril, lets not forget those early order Paladins in the past would surely sought a metal that is reachable location wise