1. It does not use HTML/JS. It is a fully "native" app (at least if C# counts as native) written with C# and UWP/WinUI2 XAML. Actually, Xbox Music in Windows 8.x had a web tech based UI; when it was rebranded as Groove Music in Windows 10, its UI layer was rewritten.

2. it's not "after"; Groove Music was largely written in 2014-2017 in the early Windows 10 days, and even its rebrand as Media Player in Windows 11 happened in 2022, and it's barely been touched since then.

Which barrier?