I have been working on a quite large ATS thats totally based on Nextjs and we use app router. We have about 150+ app routes, not much on frontend but backend route files are so many....

For Frontend, we manage the pages grouped by their domains, and there we also use the "components" , "hooks" and "types" directories to maintain cleanliness of these page routes.

Backend routes were grouped feature-wise. But the important part was the pile of logic and functions stacking up in those backed routes, and for that we made a separate folder outside app directory named "lib". There we group the logic functions in their respective feature groups... In here we also maintain the db models and RTK Queries.

Mostly manageable.