More specifically, I’m thinking about forkgram, a telegram fork. I get that open source app make their code public therefore people can use it and build from it, but I thought it allowed for creating a separate app. Forkgram however still has all my chats so it must be able to connect to telegram servers and both apps must be somehow intertwined. Since telegram is only partly open sourced, I don’t understand how this is possible and why telegram would allow this?
An app “talks” to server through some interface (what they call an API). If the interface is known, another app can use the same interface. Telegram goes as far as providing code that interfaces with their protocol and full API documentation to explain how to use it. The base app is open source too, so developers can even peek at how the official app does it for inspiration.