Pretty much every non-GUI (and GUI if it's Avalonia/Uno/etc.) .NET application, unless it uses niche things like windows registery or management API, is by definition platform agnostic and runs wherever .NET runs (macOS, Linux, FreeBSD(with caveats), Android, iOS, Windows, sometimes in browser with WASM). Most businesses which have moved away from old versions run their server workloads on Linux hosts within K8S or otherwise.

Popular applications that run on Linux are

Jellyfin: https://github.com/jellyfin/jellyfin

Sonarr (and other High Seas apps): https://github.com/Sonarr/Sonarr

Ryujinx: https://github.com/Ryujinx/Ryujinx (the kind of project that could likely compete with writing a browser in complexity)

Bitwarden (server): https://github.com/bitwarden

Garnet: https://github.com/microsoft/garnet (likely the fastest Redis implementation)

Stride3D: https://github.com/stride3d/stride

Godot (offers C# as script language, using regular .NET)

Various MonoGame and FNA games (which use vanilla .NET unlike Unity, there was even a game on Nintendo Switch with private FNA fork).

From the top of my head, I'm sure there are many others.

Discord uses electron/React native. Where did you hear it uses .NET?