-The boys were loud even after I asked them to quiet down.
-They seem upset, could you try talking to them?
-I can’t blame them.
-My mom took them shopping.
-I didn’t buy any eggs because the store was all out of them.
Them is a pronoun used to replace more than one noun, not including oneself, when not the subject (so a direct or indirect object). If what’s being replaced is the subject then “they” is used.
‘Em is used as a short, very informal version of them.
-He whacked ‘em right over the head with that baseball bat.