DX does not change anything except it eliminates need for ANT tool and assumes every developer will have DX CLI installed.
If you are familiar with DX , you will just use DX to create your project build and test and then use convert commands to create a metadata API for deployment to an org from where you generate unmanaged package .
On the git repository you can specify what DX commands a developer needs to use to deploy to the production or sandbox org or to pull into a scratch org .
For public repositories there are open source projects to deploy the code to the scratch org using one click buttons like this one
SDFX is just new way of thinking about source driven development and nothing changes at the core of it .