Student or Learner
"Students in this class are mainly girls"
in this sentence, mainly is an adverb. it places between "are" and "girls" . in the whole sentence, which word does "mainly" modify.
Also, in another sentence "Japanese are traditionally Shinto"
Shinto is a noun, how come it comes after traditionally? desperately looking for answers.
Well the adverb modifies the verb, but since the verb is to be, the adverb's meaning does describe how or what the object is, so it does feel as though you are correct. But no, it is indirect so the adverb modifies the verb, which gives more information about the noun.
The traditional place for an adverb is: immediately after the verb, or, in informal speech, anywhere after the verb, or after any other words following the verb. It can also go immediately preceding the verb, however, English is very flexible on this.