Not OP, but their description fits mine perfectly. When I write, I don't know what will come out until it does. This goes for this comment (funnily, typing this is not harder because I'm aware of the process - it just pours out) as well as the novel I'm writing. When I'm not writing the novel, but thinking about it, there are two different stages: "thinking about the chapter I plan to write" when I form the sentences, but that quickly becomes impossible because I can't keep more than a few sentences in my head.

When I'm not in front of the computer, I often abandon this process and instead focus on the larger concepts: the alien species and their evolution and civilization, the plot and the major events that need to be there. These are all concepts or scenes. I don't want to say they're "visual", they're more "concepts".

I can compare this to wanting a cup of coffee. I don't think "I would like a cup of coffee" (although that is what I might say if I have to vocalize it). It's more a feeling (wanting) and a concept (a cup of warm coffee). The label underlying the concept is secondary.

When I think of my novel in the shower, I haven't the slightest idea idea how I exactly will get to the major event at the end (because there are several major events before that which I haven't gotten to). This can only unfold when I write.

Edit: I just did the "Vividness of Visual Imagery Questionnaire" and I don't have aphantasia, but I realize that when asked about scenery that I've seen, it's easier to have a clear picture, whereas completely imaginary scenarios are concepts, rather than vivid images.