https://rolescape.io/ is a fantastic tool for GM-Prep as well as solo-rpging, everything from characters to locations to events is easily referenced. If you want the whole experience with character sheets and (virtual) dice, my answer is foundry vtt. Their module for Ironsworn / Starforged is top tier.
I’m just starting what I hope to be a long-running hexcrawl oriented game (I’ve been streaming my prep) and the one app that I’m leaning heavily on right now is a digital whiteboard app called Weje. I find it very useful to organize and gather information.
This is a very interesting question. Unfortunately, I don't have an answer (interesting or otherwise). I do my prep in a desktop text editor called Emacs with the org-roam extension, but that doesn't fit with your requirements unfortunately.
I use obsidian.md on the desktop and android for organising my campaigns, solo or otherwise! I like it because you control/keep the markdown files - they're not at the whim of a cloud-service that may disappear or jack up their prices. It supports add-ins, and I used the dice-roller one to make several oracle pages, for Mythic tables, and others. And of course to just roll dice. Obsidian is very cool, with many features!
@Gunderstank @doggoblingames I've heard that Obsidian is also an excellent choice for notes, yes. It is also used for Second Brain-type stuff just like Emacs which I use.
Welcome to the SoloRPG community!
Some people prefer to play their tabletop roleplaying games alone, to avoid the drama and scheduling issues of traditional group games, while others play solo because they're unable to find a group to play with.
Whatever your case may be, you're more than welcome here to discuss your experiences as well as ask any questions that you might have.
For an introduction to this style of play, read Why Play Roleplaying Games Alone.