In contrast to Mono with .NET Framework .exes, you can't run .NET 5 .exe files on Linux even with the runtime installed.
So while for the .NET Framework you only needed to compile a single .exe which could run on all platforms thanks to Mono, with .NET 5 you need to build one executable/binary for each platform...
~~On the plus side, this binary is basically just a small starter binary (if you don't compile in single file mode), which then loads all the real application and all its endencies as DLLs, which should be the same for all platforms.