When you call a singular a plural.

but they/them is singular cause like if you don't know the name of a person and they have a gender neutral name like Sam you'd use they or when someone loses their keys and you don't know who did you'd say "someone lost their keys"

But they and them means 2 or more people though.

not always they/them refers to an individual that you don't know the gender of or 2 or more people it can be plural but it can also be singular such as: Someone dropped THEIR wallet I should probably get it to THEM