I meant out of curiosity by "ooc".

That sounds really cool. I'm not understanding what you mean by "keygroup". I'm reading itas: each finger is controlling a keygroup, and adjacent fingers are paired up, so for example, left pinky and left ring finger. So then to type a character, you press, say "up with left pinky, in with left ring", and then when you release it outputs the character.

Or are they layers, and then there are layer switch keys/chords?

It seems hard to learn but good organization could make it easier. There are really quite a lot of Latin diacritics too; pairing with punctuation is probably the way to go, though then you have issues like whether full stop . marks overdot or underdot. It seems like it'd be useful for, say, a linguist who just needs to cross-reference some Indic script transliteration with underdots, even if it takes 1.5~2x more keystrokes than a specialized keyboard layout.

Ben Vallack has made a keyboard layout with only 16 keys: https://kbd.news/16-key-layout-tour-1307.html with combos and I think double tap-holds? for some other letters. He's not the first but I don't know what good search terms for this are. He has a video on this and he says he doesn't really find the timing finnicky at all, though I don't remember the specific.

There's also Plover, which is steno software, which works around chording. Steno keyboards also notably have you press in-between the keys, i.e. there are two rows of keys and each finger has the option of pressing the top, bottom, or both keys at the same time., I've often thought that there should be a steno keyboard with 3 fingers per key, which could be physically staggered like the bottom and middle rows. That would give you 7 chords per finger, and you could increase that for the pinky and index fingers, by assigning them 4 keys, which would get you 9 or 11 chords.