I'm not sure what there is to be at odds about here. I literally just objectively explained what is behind the gear icon. It is indeed mostly just read-only data about the app state that is more or less only useful if you wrote the app or are working with whoever did (or are writing one yourself). Again, perhaps unfortunate that it's in such a visible area and that the icon doesn't really reflect its use.
I don't work here (or write any of them)--just sharing my perspective as a longtime user. I hope you don't take anything I say in the wrong way. Again, it seems rare that someone who is brand new not to just Hubitat but to home automation in general posts here and shares feedback so clearly, so I was just glad that staff were among the first to see (and respond)!
Honestly, I'd say that description of a UI is more or less due to the model inherited from ST (which has only a mobile app for this kind of configuration, and they had to make some sort of input model that was usable on both iOS and Android). But it's also part of the whole app/device security model: an app can only access devices you've specifically selected in the app. This explains why almost every app asks you to choose devices from a list, often as one of the first inputs you see. You'd be under the same limitation if you wrote an app yourself (another reason it would be good to try stock apps before trying to write one yourself--but also because they can already do a lot, and it might save you a lot of time; feel free to ask if you're trying to do a specific automation but don't know where to start).
The language is Groovy, by the way, but it runs in an environment/sandbox similar to that of ST. Some parts of the underlying language/libraries are unavailable, but the platform also provides a lot for you on top of what you write yourself. The ST Classic developer docs as suggested above aren't a bad place to start. There are a few threads here on the mostly minor differences (as well as similar questions about where to start with development). But again, my suggestion wouldn't be to start here on day 1.
We must, as I wouldn't even call it this. I'm still curious what the use/value is of multiple logins for this particular aspect. The Portal login--where you can have as many accounts as needed--is separate, as the hub login is just for administration (so likely just you or a trusted household member). I guess things brings up another point: the web UI for the hub really is just intended for administration. Some of it isn't pretty (e..g, device pages). For "regular" users in your house, a Dashboard (either the native option or a third party option) would probably be better for day-to-day device control and monitoring--and while you can get to a Dashboard in multiple ways, the mobile app via any associated Portal login (no hub admin UI login needed) is one way.
Also, there are some threads here with cool things people have done with dashboards if you don't like the way they look by default. I agree that they editor, at least, is a bit clunky, but it works once you get used to it. SharpTools was mentioned above and may look sharper (no pun intended) with less effort, though it is cloud-based only.
Honestly, this is probably true. I attribute it to the classic "ease of use vs. power" tradeoff common in the application/UI world. Automation possibilities in Alexa work for many people but are severely limited in their power--basically, "do X when Y happens." Hubitat can do pretty much anything you want with any (supported) device. I don't doubt that Hubitat could benefit from the touch of a user-experience professional, but staff have stated in the past (not in response to this particular question but in response to many others about this kind of thing) that their priority at this point is to make a stable platform first (and presumably also one with enough power that we find it useful). The only bad news is that if you stay around long enough, you're likely to forget what you once thought was quirky. I sure have. (Again, part of why I hope they find your feedback now useful--as they seem to.)