To add references between projects (build units like DLLs or EXEs) within a Visual Studio solution, you need to open a popup from the context menu on a child node of the referring project in the VS solution tree, the child node set and the context menu have a different layout depending on whether that project follows the 2021 dependency system. The popup freezes VS so you can't keep it around, and it does six other things related to dependencies, except for NuGet which for some reason masquerades as an editor tab instead (equally inconvenient for different reasons). In the popup you tick checkboxes in a massive list of every project in the solution, then you close it. This doesn't save the changes and there's no explicit way to save changes to a csproj file because VS manages csproj files and presents them exclusively as visual forms. You can save either by "Save All", or by unloading the project which prompts you about unsaved changes and then reloading it to be able to actually initiate a build it's included in.