Download the entire source code of .NET Framework 4.8 as a .zip file. The archive includes a solution and project files that can be opened in Visual Studio. This solution will not build because it is missing crucial components such as resources, XAML files, etc. but it will be sufficient to browse the source code inside Visual Studio.

.NET 4.8 for Windows 10 May 2019 Update (60 MB) .NET 4.8 for Windows January 2020 Update (60 MB)