You have two options to reference other projects:
Project References
This is typically done using a monorepo approach and most times all projects are part of the same solution.
If you want to reference projects from other repositories, Git sub modules may be an option https://git-scm.com/book/en/v2/Git-Tools-Submodules
Project references look like this in your my-project.csproj file:
<ProjectReference Include="..\..\Modules\Core\Core.csproj" />
NuGet Packages
Here you build your framework and publish it as NuGet package. Then you reference the package (not the project) when needed and Visual Studio will download the specified version from your NuGet repository.
Two options to setup your own NuGet repository:
You can also use NuGet packages from local file system How do I install a NuGet package .nupkg file locally? (personally I'd not recommend this for most scenarios)
Reference NuGet packages in your my-project.csproj file:
<PackageReference Include="Serilog.AspNetCore" Version="3.4.0" />
Or using visual studios package manager: https://learn.microsoft.com/en-us/nuget/quickstart/install-and-use-a-package-in-visual-studio
Note that you have to add your NuGet repository feed first: https://learn.microsoft.com/en-us/azure/devops/artifacts/nuget/consume?view=azure-devops
See also