Recognizing that my own GMing methods are not those used by everyone else, it honestly depends upon the plot line that I'm running and what the end goal is. When I know what the end goal is of a story (Is it to create a long story arch? Is it to create character development? Is it to just have something to do?) then I know how I will approach it.
If it is an intro or some part of a long story arch, then I have specific points of information or occurrence that need to get conveyed and those will be planned. Example: I ran a plot line not to long ago, it had the intended consequence of a) increasing "crime/gang activity", "ousting a corrupt group" and "bringing in an FBI group" that were meant to be part of a longer story arch. The players coming into the scene (of which I limited signups to 5 people) knew that the scene was going to be potentially dangerous, depending upon their actions, but not immediately dangerous to them. I also set a time limit on the scene, what I had in mind would be done in 3-4 hours. (Never ending scenes have happened to me before, they tend to be meandering and pointless and frustrating to me.) I began the scene by having everyone describe, rather mundanely, what they were doing within a fairly large area where something was taking place.Then I blew up a Starbucks. What happened next was entirely character reactions to that, to the situation that had happened - some reacted by quickly leaving! Some reacted by leaping in to try to help people. The scene of what happened was scripted in-so-far as the thing that happened, but beyond that thing (that I knew would create the long term things that were planned) the rest of the scene was player driven.
I handle character development scenes similarly, where each individual scene is treated a bit like a book chapter - I have a specific thing meant to come up/be handled within their individual PRP, say for example something from a character's past is coming back to haunt them. The introduction scene might just be them having something happen that reminds them of that past, or they see/think they see someone they know, etc. Quick, 2 hour scene, right there, they can involve a few other PCs if they choose, if not then it's NPCs. The next chapter is a more direct interaction with that thing/person, then the next they go to confront/or spy, to learn more about what is happening. But each of these turn into little "mini-scenes" but I do put a bit more preparation work into these than I actually do the larger things, because you don't know just how soon you will need the details.
And then the "just something to do" scenes, those are a bit more broad and open. Usually a social event? Open to however many PCs want to jump in (though I do still set a time-limit, usually 4-6 hours depending upon what it is and what's happening). I set a scene, I occasionally toss out something pre-planned that is ambiance and watch in the event NPC interaction is called for.
I've been in this hobby for a long time, I didn't actually like trying to GM much of anything before playing on The Reach (which was a few years ago, but in the overall of how long I've been doing this it's not been that long ago). Around that time I started participating in these real-world "Game Mastering" competitions and befriended a number of professional GMs who gave me a lot of tips to help me get more comfortable and challenge myself doing it. So while I was on The Reach I started taking those tips and doing little scenes - I ran these things I referred to as "Choose Your Own Adventure" plots, where I would have X-number of people sign up, every person was to give me a story element for the story to be told (I think one of them the story elements were: Puzzle Box, Music, Green Light, Skull. Another one had: Mall Santa, Tinsle, Light Saber and Baby Jesus (That was a Black Friday story, which itself was an element.). But I would run these "mini-stories" for 4-5 random players, usually last about 3 hours, and found myself getting a lot more comfortable with it as well as overall good feedback from the players involved (because I would ask, that was the point of trying to improve for me.).