Is the .NET Framework 5 Source Code available online ?
It should be. The .NET 5 framework is based on .NET Core and .NET Core can be found on GitHub.
There is no .Net Framework 5. If you’re talking about .Net 5 (which is the next version after .Net Core 3), then that is being developed in the open in various repositories under the dotnet org on GitHub, most notably dotnet/runtime, which contains the class library (with a more convenient view at source.dot.net) and both runtimes that will be used by .Net 5 (CoreCLR and Mono).