Salut Jannat,

rien (pronoun indefinite / noun)   --> nothing, mere nothing, the slightest thing

aucun (pronoun / adjective)  --> no, none, not one, not any

They are not synonymous and their employment depends on the context.

The following WordReference forum discussion will be helpful for you to check out.

https://forum.wordreference.com/threads/aucun-rien.745430/

Hope this helps.

Bonne continuation.

Jim