I think it's necessary to think of 2 different uses of "ce":
1.) ce = "this" in the masc. sing. form (it's an adjective, used before a noun): ce livre, ce garçon, ce pays.
This one has different forms for plural, feminine, etc: cet homme, cet ami (special form before masc. noun that starts with a vowel) cette femme, cette ville ces amis, ces hommes, ces femmes, ces gens
2.) ce = "it," "that", a pronoun (think of it as neuter gender). It becomes c' before the verb "est": "C'est ..." (It is / This is...)
C'est l'homme qui m'a donné le livre. (It's the man who gave me the book.)
Ce sont les hommes qui m'ont donné le livre. (It's the men...)
Notice that the plural of "C'est" is "Ce sont."
We can say "It's the men who..." in (informal) English; or we can say "These are the men." Interestingly, though, the pronoun "ce" itself does not become plural in form (though it takes a plural verb).