Is in the prefix for inside?
Actually in is not the prefix in the word inside. Even though the word is made up of the prefix in- and the noun side, it’s grammatically considered a complete word on its own. Just because the word has the prefix attached to a regular word does not mean that the prefix is grammatically correct.
Words are made up of different parts, and if a new word is formed from those parts then the prefix is lost. A prefix is added to change the meaning of an existing word, and not create a new one. Inside is an adjective, adverb, or prepositions, and is its own word without a grammatical prefix.