Some of the Armor's features are magical
The context of the criteria is key in this case:
Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:
- Is it a magic item?
- Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
- Is it a spell attack?
- Is it fueled by the use of spell slots?
- Does its description say it’s magical?
So what game feature are we applying these criteria to exactly? Although the armor we choose to apply our class features to is temporarily called the Arcane Armor, it is not a single game feature, so we can't apply the criteria to it as a whole. Instead, we need to look at each game feature separately: the armor, Arcane Armor, Armor Model, Armor Modifications, and Perfected Armor. Each of can be magical or not, independently of the rest.
The Armor
Here I'm referring to the item itself, distinct from the class features. Whether the armor is magical boils down to whether it is a magic item, which should be easy to figure out from item's description.
Arcane Armor
It's not a magic item, nor spell, because it's a class feature. It doesn't create the effect of a spell, it's not a spell attack, nor is it fueled by spellslots. This holds true for the other class features too, so I won't repeat myself later.
The description doesn't say that it is magical either, so the Arcane Armor feature isn't magical.
Armor Model
Its description states:
Guardian. You design your armor to be in the front line of conflict. It has the following features:
Thunder Gauntlets. [...] as the armor magically emits a distracting pulse when the creature attacks someone else.
So this game feature is definitely magical, but in this case it is still unclear what the game feature in question is: the Armor Model feature, the Guardian model, or the Thunder Gauntlets feature?
The guidelines we are given are too vague to give a definitive answer, but I would be inclined to rule that the entire Armor Model feature is magical, because if I don't then the Thunder Gauntlets would bypass resistances to damage from non-magical weapons, while the Lightning Launcher wouldn't, and I simply dislike that discrepancy.
Armor Modifications
The description doesn't say that Armor Modifications is magical, so it isn't, but the Arcane Infusions it grants most definitely are.
Perfected Armor
Its description states:
Infiltrator. Any creature that takes lightning damage from your Lightning Launcher glimmers with magical light until the start of your next turn.
So this game feature is definitely magical. Here too there is ambiguity whether the game feature in question is the Armor Model feature or just the benefits granted to the Infiltrator model, but for consistency's sake I'd rule the former.
In Practice
So, someone casts detect magic in the presence of an Armorer, what is sensed?
Detect magic won't sense magical class features like Armor Model and Perfected Armor any better than it would sense the Spellcasting feature. However, if the armor is a magic item, that is sensed as normal, as are any magic items created via Magical Infusions.
Then, somebody casts anti-magic field, how is the Armorer impacted?
The Armor Model and Perfected Armor features are magical and fall in the spell's "Targetted Effects" section: Thunder Gauntlets targets an enemy, Powered Steps targets the Armorer, and so on for the other abilities in those two features.
The Arcane Armor and Armor Modifications features aren't magical, though the later becomes functionally useless because...
All the magic items become mundane per the spell's "Magic Items" section, including the ones created via Magical Infusions and the armor if it was a magic item.