oppen.digital is available via four different client-server protocols.
The world wide web over a secure connection:https://oppen.digital
The world wide web over an insecure connection:http://oppen.digital
Before https there was http: HyperText Transfer Protocol. It's insecure and relatively easy for someone to eavesdrop on every bit of data transferred between you and the server. Google are trying to get everyone to deprecate http in the name of privacy, on the client side that also means cutting off billions of old networkable devices from the web, on the server side it also makes an unfathomable amount of archived material unavailable.
Geminispace, instead of the world wide web:gemini://oppen.digital
Gemini is a modern protocol that's very simple and secure. It removes everything bad about the world wide web, and leaves enough to be useful and pleasant to interact with. It has critics, but these are usually knee-jerk reactions to the basic markup format used for content, and the lack of inline images.
Similar to Gemini but over an insecure connection, the same as http vs https:mercury://oppen.digital
Gemini has one big issue for older devices, the requirement to be secure means a lot of old devices cannot connect to it, it's the same scenario as if Google had their way and banished http. Mercury is essentially 'Gemini without TLS', connections are made over plain insecure socket connections. All the other advantages remain, but it's relatively easy to eavesdrop on what you're looking at, however it also means any old device that used to connect to the internet should still be able to access oppen.digital, from an old pre-Gingerbread Android phone to a 286 from the late 1980s.