The "new .NET" (which used to be called .NET Core not long ago — the whole versioning scheme is a total fiasco on their part) is relatively cross-platform, but

The "new .NET" (which used to be called .NET Core not long ago — the whole versioning scheme is a total fiasco on their part) is relatively cross-platform, but is not fully compatible with the old one (which is only available on Windows and hasn't been updated for many years). Your colleagues probably ported an old .NET project to Mono to move it off Windows. The usual thing these days is to port old projects to the "new .NET" instead, but it isn't always possible (because full compatibility between the two was a non-goal from the start). Anyways, as dvzk has mentioned, Mono is a completely separate project and is not a replacement for current dotnet. Last time I checked, an experimental FreeBSD port was available, but MS doesn't seem to care very much about any of the BSDs.