What do route groups and underscore-prefixed folders do?

Short answer

(group) folders organize routes without affecting the URL — useful for different layout shells sharing a path space. Underscore prefixes mark private folders excluded from routing: _components, _lib keep colocation tidy.

Detailed explanation

Route groups wrap related routes in parentheses: app/(marketing)/about/page.tsx still serves /about but inherits only (marketing)/layout.tsx — letting marketing pages use one shell while app/(shop)/products uses another, even coexisting at the same URL path level (only one may claim a given final path though). Groups solve real problems: separate root layouts for auth flows, applying Suspense/loading per section, organizing without polluting URLs. Private folders (_name) opt files out of the filesystem router entirely — components, utilities, fixtures sit next to the routes using them without creating accidental endpoints; the alternative $ prefix opts back INTO routing for unusual needs. Conflicts to watch: groups sharing a path must not define competing pages at identical URLs, and multiple root layouts require groups with each holding its own html/body. These features reward deliberate information architecture.

Common mistakes

• Creating duplicate paths across groups causing build conflicts.
• Not knowing _folders stay out of the router.