Hello, hello! SO. I've been told how I prep for campaigns is insane, so just know that going into this.
For one, I don't really do plot points or big bads at all. My campaigns are generally lore and location based instead. I craft a location (a city, a country, or perhaps several countries) and then populate them with different secrets and people and shops and quests or whatever the case may be. I feel that this approach gives the players a lot of freedom while not leaving me completely unprepared for unexpected turns. It gives players the opportunity to choose their own goals and create their own story. It's really great for minimizing railroading, if you're worried about that. I've never gotten any complaints like that!
Two, before the campaign even starts, I usually talk to my players about what kind of game they want to play. We hash out the setting, the scale, if there's magic or not, what kind of creatures are around, what kind people they can meet, etc. etc. I, personally, homebrew almost everything anyway, so I let my players do whatever the hell they want for this stage. I'll literally tell them, "do whatever you think is coolest, I'll make it happen". That is A LOT of prep work though, so don't be like me if you're not up for that. It's also around this stage that I subtly find out what my players' goals are for their characters. Sometimes when they're talking about their characters, they'll just outright tell you. But if not, some leading questions will help a lot when building different scenarios and opportunities.
Three, I throw my players into a starting scenario that gives them clear immediate direction. What this might be depends on what you know about their characters. I always plan my starting scenario around things I already know my players want. If one of my player is greedy, I'll put them in a bar where some shady folks are planning a robbery. If they're looking for something, I'll put a knowledgable character in an obvious spot so that they can ask for information. That kind of thing. I focus on why their characters are there. Another thing I do that's somewhat related to this: I often intertwine my player's backstories. It's semi-secretive in that I'll ask one player if it's okay while keeping it a surprise for another player. For example, if one character is looking for a magic necklace, then I might ask another player if it's okay if their character knows where it is, perhaps an heirloom or something. I don't typically have any issues with this as I try very hard to pick connections that are too funny or too cool to NOT do. Match the connections to the players' sense of style, yk?
THe best part of this method for me is that it's flexible and easy to expand on. If I have things that weren't used in one location, I can move them elsewhere. In between sessions, I can incorporate ideas that my players had or inspired without disrupting anything. It's a lot of prep work, but I find once play actually starts, it takes a lot of the pressure off and just lets you have fun playing without worrying about staying ahead.