Logically you come to the conclusion that 'one of you' is second person singular whereas 'one of them' is third person singular. Therefore, the old-fashioned usage would indeed be 'one of you are the girl' but, for example, 'one of them is the person'. In this case it's not plural vs. singular, it's second vs. third person.
Allof them are singular, since all means many, ie plural. Interestingly, "All" does not follow the above rule and is plural. Other examples that can be singulars (that are used incorrectly too often) are: The majority is here( The majority of club members are here) Allof them however suffer the same overcrowding, appalling prison conditions and mistreatment from the prison [] authorities that are the [] lot of more than 300 prisoners of opinion on the island. arabia.reporter-frontieres.org. arabia.reporter-frontieres.org.

Infact, it’s so bad that the United States government is stepping in. In a press release, the Biden administration announced it is investing $150 million to improve the

Weuse most of, many of, some of with plural nouns when we are describing part of a particular group. The clue is the definite article, the, which makes the noun specific rather than general. For the same
Providedto YouTube by Universal Music GroupAll Of Them! · Hans ZimmerKing Arthur℗ 2004 Hollywood Records, Inc.Released on: 2004-01-01Composer Lyricist: Han
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all of them is or are