As previously mentioned, Unison Devices are fully autonomous, independent, and truly alive beings. They're the logical conclusion of advancing both the power and sophistication of AI-driven Devices to the point that they themselves become sophont lifeforms. This, of course, means that the creation of a Unison Device is entire orders of magnitude more complex than assembly of other Device types.
Unison Devices aren't 'made' - they're grown and raised. Espiria's claim to fame amongst the Free States was successfully reverse engineering the method by which Belkans originally created the Unity Knights and proceeding to leverage basically the entirety of their military structure on the idea that they were able (and willing - some people tend to get uppity about the idea of giving a superweapon a soul) to mass-produce Unison Devices at scale. If you're familiar at all with 'Dragonriders of Pern,' the eponymous dragon riders are an apt analogy - except for the fact that instead of engineering dragons to deal with an ongoing natural disaster and then immediately discarding the technology to keep with the pastoral values of Pern's nascent society, Espiria was a technologically advanced civilization at war with no particular desire to pretend at being anything else. As such, they both continued to engineer more and better
dragons Unison Devices as well as devote a lot of time towards mitigating their inherent drawbacks until the point that Espiria's military had something on the order of 40% Unison Device adoption. This went a long way towards papering over the fact that they lacked the raw numbers of some of their allies or their enemy and also allowed them to quickly recoup losses - Espirian Unison Devices, especially the later generations, traded a higher potential ceiling for both a broader base of potential partners and the capacity to jump partners as necessary - including if the Mage wished to retire after a tour of service but the Device
didn't.
(This is one of the reasons why Mariposa came fully loaded with all the training tools that Taylor could ever want. While Taylor's specific situation is very atypical and likely would've produced a few raised eyebrows if Mariposa still had a command chain to report back to, pairing a fresh-faced Espirian recruit with a Unison Device and letting them start emotionally bonding to each other through the process of basic training was a common thing.)
Entering Unison has an exponential effect on the combined partnership's total power based on the aptitude for Unison and the compatibility between partners - both an emotional/mental compatibility as well as their physical synchronization rate which is in turn a function of the Device's makeup and the Mage's developed magical capacities. Earlier generations of Unison Device required a lot of manual intervention on both sides of the equation to get the synchronization rate to adequate levels; later generations gradually became able to perform those interventions automatically by leveraging resonance theory, which essentially proved that prolonged positive contact between Mages caused them to gradually attune to each other, but there's still a barrier to entry for being able to synchronize at all - e.g. some people are just fundamentally incompatible and therefore the process can't ever begin. Further, if the starting synchronization rate is low, the amount of time needed for it to naturally come up to an acceptable level was generally prohibitive compared to the amount of time needed to find better candidates for partnership.
Successful Unisons always produce 'physical bleedthrough,' a blending of the features of Mage and Device. Lower levels of compatibility produce slight changes, higher ones more striking - this is of course why Unison Taylor has blue hair and gold eyes instead of her normal black hair / brown eyes. In such situations where the Device has to act autonomously because the Mage is incapacitated, physical bleedthrough will go farther representing the Device's increasing levels of control, up to the point of assuming the appearance of the Device totally as the Mage's body is shifted into a safe internal space for rest and recovery surrounded by a Jacket-like protective layering composed of the Device's appearance. Ancient Belka called this a 'Unison Accident,' but it's really a feature - the Device taking over for you if you're out cold is vastly preferable to the alternative of a forced disengagement or simply being left unconscious at the mercies of whatever did you in, after all.
Between the emergency repairs needed to deal with The Locker's effects as well as the fact that Taylor's magic was a blank slate, she started off with an extremely high physical synchronization rate regardless of what it 'might've been' or 'should've been' in another world or timeline. It's not a great metaphor, but you could consider it a kind of situation where rather than creating a key to fit a certain lock, the lock was remodeled to fit a certain key; the result is the same (a matching pair) but was arrived at from the opposite direction. Since this kind of thing relies on picking up someone from a null magic world with both the necessary power and aptitude for Unison, it's not something that comes up often. (Note that this is a purely physical occurrence; it does not mean and shouldn't be interpreted as some kind of 'Taylor was Mastered at the start of the story' suggestion - because she wasn't.)
