I think one of the things that isn’t explained that well is what an autocaster actually is. The answer is unfortunately that it varies quite a lot. In Weber’s case, since he doesn’t actually use one in combat, it’s more of a (quite expensive) toy. This was covered a bit in Weber’s side story that he likes to play with them to try to understand them, so what he has is called a General Autocaster. It can be loaded with different spells, that he can then cast, and in his case, his is essentially like a special module built into a phone. They do require some degree of hardware.

What Amy and Tyler are referring to though is that spells are loaded onto them externally – you wouldn’t typically enter a new spell manually, and on an autocaster like this one, there wouldn’t really even be an interface to do that – one like this actually can build more complicated spells, but only out of existing building blocks, and it does so incredibly inefficiently. As the building block for what Mium wants to do wouldn’t exist, there shouldn’t really be a way to enter it. That said, Mium’s proficiency with devices like this tends to surpass reasonable expectation for reasons that are probably obvious to the reader, and that Tyler has some suspicion about. I’ll note that while Tyler does not seem to know what Mium is, he suspects something is amiss with him.

Tyler is beginning to take some hits to self confidence, as most of his recent missions have ended in absurdity – people that can teleport, people that can drop him into a realm he cannot understand, etc. That said, he is an exceedingly talented mage by any normal metric – he’s just a moderate large sized fish encountering ever larger ponds. Part of the reason Elmon dumped him here rather than simply trying to escape him here is that someone like Elmon can see the threat the Tyler represents – this time Elmon had a trump card that Tyler was unable to counter, but someone with Tyler’s level of aptitude is going to be able to counter this once he knows how, and would be significant trouble for Elmon if that came to pass.

How I represent this place is probably going to change, but narratively I will chalk that up to the fact that what it looks like largely depends on what the characters that are there can perceive – we already say that a little with someone like Elmon being able to perceive far more instantly realizing the Mium was not what he appeared, even if he didn’t really understand what he was seeing. Tyler is beginning to adapt to being able to see more here, and we are seeing this from his point of view, but as he started with no real comprehension of what was going on, this place mostly just looks like an empty void to him. Someone like Mione or Magnolia would see far more. Mium’s warning to Tyler isn’t far off though – Magnolia has far more experience than Tyler in perceiving this sort of data, and we’ve seen that she can only really absorb in small bursts and quickly tires out and gets a headache when doing so. Someone like Tyler that doesn’t really know how to filter this data out would quickly be overloaded by the data if he could see too much of it.

When Mium refers to the spell not being perfect, that’s fairly typical – no magic characters use is perfectly formed, but an Eidos key can complete the spell as long as its close enough. It’s sort of like autocorrect. In this case, Mium largely just set up a spell that alters some data, and is entrusting that Tyler’s ability as a natural mage will carry him through being able to use the spell – Tyler is both a natural and innate mage, and exceedingly good at magic – Tyler isn’t the character in the series with the greatest ability to complete incomplete spells, but he’s in the top few there. While essentially all humans can use magic, the ability to use rely on ones Eidos Key is generally considered part of what makes “talent” (as well as ones capacity and ability to gather mana). I will note that how much these are truly innate abilities vs. trained abilities is a matter of some debate in-universe.