The word ‘all’ is not a preposition. It is primarily used as a determiner, pronoun, or adverb, depending on its use in a sentence. As a determiner, it refers to the whole quantity or extent of something, such as in the phrase ‘all people.’ As a pronoun, it can stand alone, as in ‘All are welcome.’ When used as an adverb, it can modify adjectives or other adverbs, like in ‘She is all ready.’
In contrast, prepositions are words that show the relationship between nouns or pronouns and other words in a sentence, such as ‘in,’ ‘on,’ ‘at,’ and ‘with.’ Therefore, while ‘all’ can be versatile, it does not serve the function of a preposition in English grammar.