This is the Unity project for the SHSO remake, you can use it to connect to your own self-hosted instance but not the public server for security reasons.
You will also need to copy the AssetBundles folder from Assets into the output _data folder, and edit the server.xml file located in AssetBundles/Configuration to point at your self-hosted server (127.0.0.1 should do the trick, unless you are hosting the server in a VM, I think).
For the server files, go to SHSUnderground/shso-server.
For any help with any in-game issues go to the official server below:
Go to #development-only in the community server below:
Make a fork and PR.