The way I code now after 10 years: Use one big file. No executable I'm capable of writing on my own is complex enough to need 50 files spread across a 3-layers-deep directory tree. Doesn't matter if it's a backend, a UI, or what. There's no way your React or whatever tutorial example code needs that either. And you don't gain any meaningful organization splitting into files when there are already namespaces, classes, structs, comments, etc. I don't want to waste time reorganizing it, dealing with imports, or jumping around different files while I code.

Oh, there's some custom lib I want to share between executables, like a Postgres client? Fine, it gets its own new file. Maybe I end up with 4 files in the end.