Obsidian is the best app for dungeon masters (images in post)
It's genuinely so good. You get a cool graph view where you can see all the story connections. And with a few plugins you can get your own fantasy calender and interactive maps. There's also plugins to help you track initiative, fantasy content generators. You can go as wild as you want.
On it's own obsidian is basically creating your own wikipedia, but there's so many more tools other than the normal note writing. + it's fully free
With the worldmap if you press on one of these icons you go to another article, for example on one of the cities you go to the city map. On the stars it's cuz a special event happened there and goes to the note about that event.