Private servers are typically created using server emulators. One of the most prominent tools is the GMDprivateServer
A private server is an unauthorized, third-party host that mimics the official Geometry Dash servers. When you log into "GD11," you aren't logging into Sweden; you are logging into a server run by a fan in their basement—or a sophisticated collective of modders. geometry dash 11 private server
For a look at the process of setting up and interacting with these community-run environments: How to make a Geometry Dash Private Server [TUTORIAL] YouTube• Nov 27, 2015 1 update features? Private servers are typically created using server emulators