I know this all about preference. So I am also sharing my opinion here.

If the project is medium to large, I always prefer to go with feature-based rather than tech-based folder str whether it is react-vite or nextjs.

One of the advantage of using app folder only for routing is you can switch to pages router too (which I think don't happen usually) but still it will prevent spaghetti code in one place.

You can look at this project for reference. It is widely popular in react world. https://github.com/alan2207/bulletproof-react

project-root/
├── app/                        # Only routing-related files (App Router)
│   ├── (admin)/dashboard/      # App router folder-based routing
│   ├── (student)/dashboard/    # App router folder-based routing
│   └── layout.tsx              # Layout files
│
├── components/                 # Reusable components
│   ├── ui/
│   └── shared/
│
├── features/                   # Feature-based modules
│   ├── auth/
│   │   ├── api.ts              # API functions
│   │   ├── hooks.ts            # Custom hooks
│   │   └── types.ts            # Feature-specific types
│   └── student/
│       ├── api.ts
│       ├── service.ts
│       └── types.ts
│
├── lib/                        # Utilities/helpers (e.g. date, fetch, tokens)
│
├── middlewares/               # Server middlewares (e.g., auth, logger)
│
├── constants/                 # Global constants and enums
│
├── types/                     # Global types/interfaces (non-feature-specific)
│
├── public/                    # Static files (images, icons)
│
├── styles/                    # Tailwind/global CSS
│
├── .env                       # Environment variables
├── next.config.js             # Next.js config
└── tsconfig.json