C'est un grand appartement=it's a big flat
Comment est cet appartement? (How is that flat?). Il est grand. (it's big).
Qu'est-ce que tu penses de ce pull gris? (What do you think about this grey pull-over?)
Il est moche.(it's ugly)
Qu'est-ce que tu penses des pull-overs gris?
C'est moche. (it's ugly)
Il is used as a defined personal pronoun (the same way as you use "he" or "she" for people in English, we do the same thing for things in French). Ce (c') is used as a neutral pronoun.
So, you use "il" if you really want to talk especially about THIS thing.
This pull-over : it's ugly
Ce pull-over : Il est moche
And you use ce(c') to talk about it in general
grey pull-over : it's ugly
Les pull-over gris : c'est moche.
In a sentence as "It's an ugly pull-over", we always use "c'est" for things such as people, because it's a demonstrative structure.
"C'est un pull-over moche"; "C'est une jolie fille" (it's a pretty girl), but "Il est moche" (it's ugly); "Elle est jolie" (she's pretty).
We can't say, as in English, "She's a pretty girl", we always say "it's a pretty girl" (C'est une jolie fille) and it works all the same with things. There are no grammatical differences to talk about things or people in French : the thing is a masculine word? Talk about it as if it was a boy. It's a feminine word? Talk about it as if it was a girl.
Just pay attention. C'est beau/c'est moche/c'est grand....(it's beautiful, ugly, big) for example, can not be used for people.