You can technically use Voyager on desktop and mobile. You can access the web client here: https://vger.app/

Voyager is designed to work on basically any kind of device. It is available as a web app or on most of the major app stores, including Flathub for Linux use on desktop if you want.

https://github.com/aeharding/voyager

While Thunder is primarily designed to be a mobile app (and only Android/iOS builds are officially distributed), I have certainly built it from source to run on my Linux desktop just to try it. It's just a Flutter app. The Flutter documentation outlines how to build a Flutter project for Linux.

https://github.com/thunder-app/thunder

But you really have to ask why you want a native Lemmy app instead of just using the website on desktop. There some convenience factors for mobile apps, but I can't really think of why someone would not just want to use the website when using the platform in desktop. Perhaps they just really want the same exact UI across every device or something.

I think that's the question OP is asking too. You could do some convoluted shit to get an emulator running and just use whatever phone app on both I guess.