If there's a sequence of AUG in an RNA, then an RNA-dependent RNA polymerase can copy that to UAC in a newly synthesized RNA, because those bases "fit" in base pairs. Then the UAC in that RNA can be copied into AUG in a new RNA that's identical to the original one.

A ribosome can read the exact same AUG sequence and insert a methionine into a protein.

One method is essentially just straight copying while staying in the same language, while the other is translating to a different language. That's why the process of making proteins is called translation.