Prepositions of time + adverbs?

Hello,

I have the following question regarding prepositions:

In grammar texts it is generally stated that prepositions like "for", "to", or "by" always must be followed by a noun or pronoun, like in the sentence "I did this for him", where "for" is a preposition and "him" is a pronoun.

When I thought about this twice, I found the preposition of time "since" like in the sentence "I am working on it since yesterday." If I am right, "since" is a preposition of time, but "yesterday", in contrast to what is normally said, is no noun or pronoun, but an adverb of time. Because of this finding, I now would like to ask whether:

I am right that "since" is a preposition?

"yesterday" is an adverb of time?

It is true that in the given example a preposition (since) is not followed by a noun or pronoun, but by an

adverb of time (yesterday).

If yes, is "since" an exception and in all other cases prepositions are followed by nouns or pronouns?

Are there other examples in which a preposition is not followed by a noun or pronoun?

I am looking forward to your answers.