As an alternate approach for .NET desktop apps, Electron.NET is a variant of Electron that adds a dotnet backend. Build your UI in the HTML/CSS/JS flavor of your choice and then implement the functionality backing that UI in C#.
Electron.NET is relatively new and I just yesterday published what I think is the first boilerplate sample for it (using React/Typescript/MobX)  (note: my sample app currently looks terrible because there’s exactly zero CSS applied, I was hoping to get some of that added later this week having gotten the functional details working first. The demo in the Electron.NET repo replicates the stock Electron demo, if you’d like to start your explorations with pretty first).
Serious question as a .NET developer...
Why? What's the advantage?
Second thought would be to just go typical client-server, which is what I think I'm basically seeing here?
The advantages aren't clear to me.
Avalonia  is a cross-platform XAML-like UI library for .NET that supports Windows, Linux, and Mac via their native UI toolkits.
Isn't this what Electron is for? Why do this?
Is there anything similar for Java and electron?
DotVVM has Electron support. Have you looked at it?
or just like, use Electron and cut out the dependency on .NET