I have several head canon theories that:
(1) When Balthasar arrived in the destroyed future, he spent a long time wandering and trying to gain his bearings. This included exploring various domes, finding various goods, and he began accumulating things he would need to build his time machine (Epoch) and Keeper Dome. Some of these goods may have been too much for him to carry all at once, so he decided to safeguard them for a later visit to retrieve them. Much like pirates would bury treasure on seemingly deserted islands with the hope of later retrieving it.
(2) Balthasar didn't actually build them. Could have been similar tech, or may have been directly based around technology reverse-engineering from salvaged Zeal tech. We know from the Southern Ruins in Medina and the Zeal Ruins in Radical Dreamers that there are still remnants of Zealian civilization scattered about, so it wouldn't surprise me if Zealian tech had been reverse-engineered at some point. Schala's pendant was almost certainty unique, but perhaps the tech (energy field? some sort of electrical charge?) and mechanics for the pendant were duplicated, at least to open the doors.