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.