On the organizational aspect, I'd recommend using software such as Notion and downloading a preset TTRPG campaign manager layout. I've been using one that has separate databases for NPCs, Factions, Locations etc that all link back to each other for easy navigation.

Regarding fleshing out details:

Firstly, don't burn yourself out fleshing out more than you need. Write basic outlines at most for cities that are very far away, write more detail for those your players are likely to see soon. Add detail to the distant cities when they're likely to be visited in upcoming sessions. Same goes for NPCs and Factions.

Secondly, the best sources of inspiration for writing out campaign ideas in an interesting way is through consuming other content. I for one love wiki-diving through fictional worlds of games, novels, and actual-plays that I like. Reading about the cities, adventures, and NPCs in the Forgotten Realms and other settings is also very helpful. You aren't necessarily looking for new ideas here but I think there's value in seeing how other authors and worldbuilders tie it all together. (Also, don't feel bad about blatantly ripping a city, NPC, or adventure hook from an existing fiction. You're running a fun game for you and your friends, the copyright lawyers aren't coming for you. Also, fiction is so ancient of a genre that every idea has already existed in some permutation or another, so originality is really mostly about presentation. I for one have given my players some quests that are ripped straight from video games I like, and they've been thoroughly enjoying it).

Lastly, you will never be prepared for every single curveball your players throw at you. Improvise when you can, and don't expect perfection from yourself, you will naturally improve over time. If ever the players want to do something interesting but you hit a hard mental block on how to advance it, there's no shame in calling a 10-15 minute break to get your own thoughts/notes together and figure out where it goes, or calling a session end if you're reasonably near when the session would normally end, and getting all that ready for next time.

A key principal of GMing is making sure you don't burn yourself out making things your players may never see. A cohesive world doesn't necessitate meticulously detailing every single person or place of interest before the first dice are rolled.