I’ve had projects where it’s just one big .tex file including all the macros and content, and projects where there’s a whole file hierarchy of chapter subdirectories and layers of \inputs and macro files and… It can sort of get out of hand for me, where it makes sense in the moment and then looking at it a week later it’s a puzzling labyrinth of files.
I guess it depends on the nature of the project, but do you have a go-to way that works for you in most cases?
The major TeX format habit I’ve picked up isn’t about file splitting, it’s about inside a file: I put a newline between sentences.
If you’re revision-controlling, it makes it …actually work…, and if you’re not (which, despite my best intentions, is almost always the case) it makes it much more navigable than conventional paragraph layout.
As far as file layout, I lately have sections developed in separate labeled plaintext files starting from the sketchy notes/outline/code snippets/etc. I took about whatever I’m writing about with a minimum of formatting, then integrate those parts into a single file when it’s time to start finalizing. It’s sort of a compromise position between a complex project with multiple inputs and spread out formatting that might interact in annoying to debug ways, and a giant single file. It doesn’t necessarily make sense in the abstract, but it’s been working for me.