These are the offline installers, portable ZIPs, and deployment MSIs for Paint.NET.
The offline installer may still need to download some prerequisites if you're on a Windows 7 or 8.1 machine that is not up-to-date. You do not need to have .NET installed, as it is included as part of the app ("self-contained deployment").
If you are installing/deploying the MSI on Windows 7 or Windows 8.1, you will need to make sure the prerequisites for .NET are installed, which you can find information about at https://docs.microsoft.com/en-us/dotnet/core/install/windows?tabs=net60. You will also need to have the Platform Update installed on Windows 7 SP1. You do not need to install .NET itself, as it is included as part of the app ("self-contained deployment"). More documentation for the MSI, including configuration properties, is over at https://www.getpaint.net/doc/latest/UnattendedInstallation.html
(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.