Is this stuff being done by solo unicorns (frontend devs with good color + design instincts?). Or more often by teams of at least a few people.
Is it a quick process that's often right the first time, or am I looking at months of design iteration and testing when I see a UX that looks even 'nice'.
Speaking broadly, good design reduces friction. Design is a methodology to solve a problem. Maybe there is a form that needs filling out, or an important message that someone needs to know. Good design makes the form easier to fill out and submit, and the message easy to read and understand. Bonus points if there's a little bit of joy to experience along the way!
Different problems require different solutions. Sometimes they require large teams, especially when user or customer research is important. Sometimes small teams or individuals can get things done faster, because they communicate well with each other. Still other times, a single opinionated outsider can move the needle with management far more than an entrenched internal team.
What you are looking at when you see a user experience that looks 'nice' may have happened by any one of those avenues. With the best design, you may never even know.
Design is a job. Good design takes work. That work happens in the infinity of ways that any work happens. A solid understanding of the problem domain makes the solution obvious, and it's a designer's job to gain that understanding, and work out solutions.
Are you a 'designer' in the sense of UX, or visual design?
I (sort of) get how usability happens, but have less experience on the visual design, layout, color, graphics side.
Like are you able to sit down in an afternoon and convert a page with working UX into a consumer-ready interface? Is there standard prep that goes into this like design systems or theming?
Coming up with that visual language is more work. Typically companies like to be unique (but not too weird) so while you can identify "best practices" among larger brands, most of your time is identifying what makes them different, and finding the best way to highlight those features visually. There tends to be some back and forth with this, and there are a lot of feelings to navigate, especially if the customer has strong preferences about how things look.
Color, layout, heirarchy, history and perception are all part of a good design education! It's worth your while to find art and artists that you like, and find how they solve problems. Paul Rand and Massimo Vignelli are two significant personal influences, and they have written extensively (or had things written about them) about their process and thinking.
(Edit: I spelled Massimo Vignelli's name wrong, oops)
not sure if this is exactly your wheelhouse, but do you have a sense of the time + expertise that goes into the design system of a consumer app at launch? Like are most apps using an agency, a freelancer, people on the team?