These are the offline installers, portable ZIPs, and deployment MSIs for Paint.NET. The web installer is also provided, which will download and run the correct installer for any CPU and supported OS.
Release notes: https://forums.getpaint.net/topic/121576-paintnet-50-beta-build-8395/
Please note that the system requirements have changed for 5.0. Windows 10 v1809+ or Windows 11 are now required. Windows 7 SP1 and Windows 8.1 are no longer supported. A 64-bit CPU and 64-bit OS are also required: 32-bit x86 is no longer supported.
(Friendly hint: If you don't know what an MSI is, then it's almost certainly not the download you want.)
The "source code" ZIP does not contain any actual files or source code, it is just included automatically as part of GitHub's "releases" functionality.