A folder is a street; only a page.tsx opens it to visitors. Parentheses are invisible.

In the App Router the folder names build the URL, but a folder only becomes a reachable route once it holds a page file. Wrapping a folder name in parentheses makes a route group: a way to organise files that adds nothing to the URL. So a page inside (shop)/cart is served at /cart, and a page directly in (shop) is served at /.

Read it once, then a question tests whether the trick stuck.