My answer to the last part? All of the above. But let me dig deeper.
It took me a long time to learn to write
more economically , to lean out my flowery prose, and tighten my work. I knew the genre standards, and I was developing a strong understanding of plot, but I still wrote too long and too cluttered. Now? Now I can cut entire paragraphs without so much as a blink because I know when I'm going overboard or adding words that aren't needed. This is a skill that often takes time, practice, and a willingness to slay your darlings--the right darlings. (I could STILL tighten this paragraph!)
How else can you control word count?
I use math. I'm drafting a historical fantasy novel, and while I know I might need to add a bit to the 2nd draft, I'm aiming for 100K. Historical fantasy can often be longer than a lot of books because historical + fantasy (and I add a little romance) = a lot of worldbuilding. That said, I know not to bloat the page, and to get from one plot point to the next in a concise fashion. How?
I divide my books into 4 acts (still based on the three-act structure), but this gives me goalposts to reach as I draft:
- the first act turn
- the midpoint
- the second act turn/climax
- the end!
Divide 100K by 4 and that means I have 25K words to get from point to point.
I even do this for novellas and short stories. When I first decided to write a short story in 2017, I had no idea what I was doing and was pretty sure I would suck at it. But, I did know plot, and I used those goalposts to help me craft a few shorts and novelettes. I managed to win a couple contests, get published in a book with Diana Gabaldon writing the forward, and one of my shorts was published twice, once in the US in print, and once in London in audio. One of my novellas came in 2nd place in the New England Reader's Choice Award as well, and I've edited short stories for some very talented authors who trusted me with their words. All of this after someone, back in 2017, almost had me convinced that I couldn't write a short story using my method.
So yes, I prepare for length beforehand. I know going in what type of story I'm writing thus the needed word count. However, I've certainly had a novelette become a novella as I was drafting, and I've even had a novella want to be bigger. Maybe not a full-grown novel, but more ;) I'm turning a previously published 20K word novella into a 45K word novella, and now it's book one in a trilogy. It's been a lot more work than I imagined, but thinking bigger and having an overarching plot has helped. For me, plot is integral to my process. Sometimes, a story (like this trilogy) needs more planning, and sometimes, like for my historical fantasy, I just need those goalposts or major plot points/story tent poles in my head as I work. But ultimately, my understanding of plot gets me through any story.
In this way, writing a novel vs shorter works isn't very different for me. I just have fewer words to get from point to point, so my creativity is tested, which is a challenge I find fun.
Ultimately, every writer is different. We all have to figure out what elements of the writing process work for us and which ones don't, and then adjust accordingly. For so many writers, figuring out their writing process is a battle, but know that if that's you, you're not alone. It can take a lot of trial and error, but you'll get there.