Talking to a friend, and watching some videos for GMs talking about running games, I've come to the conclusion that there are three types of content that people use when they're running games. These three types of content also more or less boil down to three general styles of running games. Now this style doesn't mean you know everything about how a GM runs their games. It will give you one facet of their game style though.
The Three Types of Content
The three types of content are: 100% Custom Content - or Boutique Content as Matt Colville calls it, 100% pre-prepared content, and the hybrid approach of customized pre-prepared content.
100% Custom Content
A GM who runs using 100% custom content is generating everything fresh for the game they're running. This could be in a pre-set world or not, but the main point is that the adventures you run, the NPCs you encounter, and the stories you become part of are generated for the game. Odds are no game is actually truly 100% custom for the game - we all have our tropes and trends we fall back on or characters we really like that appear here and there - but the vast majority of the game is custom made for that game.
100% Pre-Prepared Content
A GM who runs using 100% Pre-Prepared content is running content made before the game. I say pre-prepared instead of module because at one point the content could have been custom. For example, I ran a 100% custom super hero game back in College. I have enough of my notes that I could run that campaign again for a new group of players. The content is pre-prepared, but it's not a bought module or paid for adventure. Like with 100% Custom Content, odds are there will be some unique changes to the game. The players are going to go off the rails. They're going to prompt some type of new interactions or necessitate a new NPC. But in general, the vast majority of this is pre-prepared.
And finally we have the hybrid. The thing here is that the GM is take pre-generated content, but is going in and making deep modifications to things. NPCs are being replaced with new custom NPCs that fill the same role but have their own twists and things. Other aspects are taken, bent, broken, removed, or replaced to fit what is needed. Maybe the first 1/4 of adventure A will be run, but instead of being finished it will go into adventure B, C, or into custom content.
Which is the Best?
Honestly, whichever one works for you. I have nothing but respect for GMs who can run 100% Pre-Prepared Content because it is not something I can really do. Like, I can run from a book if I need to sure...but with every session it is probably going to go more and more off the rails. Part of the reason for this is because I simply never really developed the skill set to do this. My entire career as a GM has been custom content. At most I can take ideas from pre-prepared stuff.
I've also had fun in games run by GMs doing all three. None of them is the best. It all depends on the group and how it is run.
So find which one works for you, and have a blast!