I'm still in the world and character building phase. Sketching landscapes, rooms and such, writing little short stories to form the characters (my idea is that they have a "memory", don't know if it works in the end).

As for now I'm completely analog, I bought a nice notebook and do everything in it. But when time comes I think I will go with Obsidian as I already use it for my other notes/projects. I think it will be very powerful with the graph-view regarding relationships etc.

Obsidian is not FOSS though, and there are alternatives. I just hadn't the time to look at them.

Edit: Obsidian can be used for free when you do not use their sync features. I use it that way and just have it in a Git-Repo for accessing it on different devices. Obsidian notes are in Markdown so you also do not have some kind of vendor lock, it's just about the links between the notes.