everything about them is different, you could even say they are from different frameworks. pages is the old one, app is the new one.

You don't even need to think you pages are

Just use the app router

Always

yeah, the main difference is just about modern and old version.
for deep understanding, you need to know core concepts in Next.js such as Routing, rendering method, data fetching, layout, error handling, etc.
recent projects don't almost use pages router.

thank you, @joulev , @Great golden digger wasp , @Rex
I will try to learn more.
if I have something to need, I will ask again.

absolutely