what ui library are most .net applications created in the business world today using? what is microsoft telling people to use?

There's no such thing; it depends on the app that people are building, their use cases, and platforms.

Case in point: both Teams and VS Code: both web view wrappers.

Some trading shops are still using WPF and some are even using Windows Forms apps still.

I'm amazed people are still using WinForms. When I was going around tables at a job fair in college, 13ish years ago, one of the questions for me from representatives for a large bank was about if I've used WPF. I said I don't use it, and they basically told me I should get with the times.