All you need to know about the the shift to UTF-8 for the windows API is here:
https://learn.microsoft.com/en-us/windows/apps/design/globalizing/use-utf8-code-pageWindows internally still uses UTF-16 (for legacy and performance reasons), but they upgraded all of the old -A functions to UTF-8, and also introduced -A variants of functions which previously only had a -W option.
A bit of the history. Microsoft was very early on with the unicode integration and went for the 16 bit route, because it is easy (and more importantly, very fast). Problem was that it turns out that 65k codepoints is not enough to archive internationalization (thinking about the fact that chinese already has over 100k, it's amazing how MS did not see that coming), so MS went the route of adding unicode planes, basically repeating the same problem of codepages that unicode tried to solve.
The rest of the world looked at that and thought that this is quite stupid, so they chose to go a different route, which was UTF-8, it has many advantages, mainly that it supports up to 32 bit codepoints, therefore every character can be uniquely represented, so unlike UTF-16 it actually solves the problem of codepages instead of repeating it, as well as being compatible with 7bit ANSI strings, making it a perfect fit for backwards compatibility.
This is why this was the major standard adopted by the Web, and as today most of the computer usage is using web applications, this became the dominant standard.
So in 2019, Microsoft decided to also enable UTF-8 in their API and add all the missing -A functions as in the previous years they opted for only the -W versions.
Today most of Microsoft products are using UTF-8. If you create a file with notepad, it's now UTF-8. .Net Framework and .Net Core have been switched to use UTF-8 by default. Even the Document format of Microsoft Office now uses UTF-8 by default.
UTF-8 is the de-facto standard encoding around the world now. The only thing where I found that Microsoft still does not use UTF-8 is for console applications. But thats for backwards compatibility, because Windows needs to be backwards compatible to the 90s and probably even earlier. But everything new that MS made in the past 5 years or so now uses UTF-8