Welcome @DevGiuDev!
I use a relatively flat structure. One notebook for task management, with sub-notebooks for projects (one home note for each project), next-tasks, and references (work meeting notes, people, etc.). I separate personal projects, work projects and an archive for old ones, but that's it (some would probably prefer more areas at that notebook level). One notebook for PKM with sub-notebooks for fleeting, permanent (general), literature and concert notes. I use inline tags and frontmatter for information retrieval and task management, and try to link related notes as much as I can. Templates help maintaining a consistent structure (generate note IDs, create project notes, populate the frontmatter, etc.). AI also helps.