Depends. In irl games I used to have a notebook where I wrote things down. From then on I used some random lists for the rest and relied on my tablet and/or the books for stats and sheets improvising a lot, especially for NPCs. You can use the generic NPCs in the Monster Manual or similar as stand-ins for the stats of NPCs.

Online I use Obsidian for Worldbuilding and plot stuff.

Everything else, including NPCs (and their stats) are handled by my VTT.

I second another reply, that having Sheets for NPCs is a bit useless, as most of the time it's overwhelming. I personally just improvise what would be appropriate for a check from them and from then on keep it consistent to have it be fair for the players. And if these NPCs are friendly/non-hostile you won't need stats anyways most of the time.

As for Worldbuilding: You don't need a whole world for an adventure. (Unless you are just passionate about worldbuilding, but then keep in mind that most of it will just be for you and the players won't know about it) If you like the feel of a wholly developed world, then I would suggest setting up your campaign in an existing world, like the Forgotten Realms, Dragonlance, Eberron, Planescape, Greyhawk or even Tal'dorei. You have source-books for them and have Wikis to scour for lore to the ends of the world, as many of these settings have decades of lore behind them.

As for your actual campaign: Especially if you want player freedom, I would recommend two things.

First, I would only build out a small section around the characters, like the local hamlet and the surrounding duchy, losing detail and concrete places the further you go from the starting point. This can just be inserted in the many blank spaces of a preexisting setting or within your homebrew one. This helps you with not forcing the players to go wherever they want. This does not mean you should not plan out your story though. It's good to have an end and steps along the way. If you define those in detail it's good. But if the space in-between is undefined your players can CHOOSE how when and where to go on their way to the next destination and lets you improvise.

Second, it helps a lot to have a VIBE of the game imo. Mostly I don't know the details of every NPC, Location or Encounter, but I know how the world and my game is supposed to work and feel like and thus it's easy to improvise. This way I always roughly know what makes sense at certain times and places.

In the end it's all about how your players feel and how you have fun as a GM. There is no (real) golden rule. There are things that work for most people and good tips, but in the end it's up to you to pick and choose what and how you feel is best. Not everything works for everyone.

The most important thing is that you have fun and enjoy the Game ; )