The way i understand this without using all that grammatical jargon, is that when the sentence at any other point reveals the person's (or groups) gender such as it did in this one with "garcon" and then "il" then you use "C'est" (or "Ce sont"). If you are talking about anything else that involves a person or people then you use "Il, Elle, Ils, Elles" as in "Il est calme"! (no secondary gender reveal)
As it states at the beginning of the lesson: The basic rule is that you must use ce when être is followed by any determiner—for instance, an article or a possessive adjective. Note that c'est should be used for singulars and ce sont should be used for plurals.
C'est un homme. — He's a man. / This is a man. / That is a man. Ce sont des chats. — They're cats. / These are cats. / Those are cats. C'est la fille. — She is the girl. / This is the girl. / That is the girl. Ce sont les femmes. — They are the women. / These are the women. / Those are the women. If an adjective, adverb, or both appear after être, then use the personal pronoun.
Elle est belle. — She is beautiful. (Or "It is beautiful.") Il est très fort. — He is very strong. (Or "It is very strong.")
Not sure if it's a general pattern but looking into your examples, I think that C'est/Ce sont indicates merely the existence of a person/animal (He's [subject]/They're [subject]/This is [subject]/That is [subject]/There are [subject]/Those are [subject]) and to describe qualities you have to use "Elle est/Il est" (She is [quality]/He is [quality]). Am I right?
If so, "C'est un garçon et il est calme" is especially useful to exemplify it since the first phrase ("C'est un garçon") simply indicates what he is and the second phrase ("il est calme") describes a quality of him.
This is incorrect because et is in there so that makes it "this is a boy AND he is calm"
The thing about this question is that C'est means it is/this is and by having il (he) then there wouldn't be a point in having un garcon because you have already established that the person is male
"c'est" can also mean "he/she is" in context. "il/elle est" and "ils/elles sont" change to "c'est" and "ce sont" before a modified noun, that is, a noun preceded by a modifier. A modifier can be:
- an article: un, une, des, le, la, l', les
- a number: un, deux...
- a possessive adjective: mon, ton, son, ma, ta, sa, notre, votre, leur, mes, tes, ses, nos, vos, leurs
- a demonstrative adjective: ce, cet, cette, ces
These articles go into more detail and are worth a read.
Also, the Tips and Notes for the Gallicism skill (click the lightbulb icon when you open the skill) give more information.
by having il (he) then there wouldn't be a point in having un garcon because you have already established that the person is male
But "il" could be referring to any masculine noun, not necessarily a boy. It could be a man, a tree, etc., and we don't know whether we're going to describe him/it with another noun (he is a boy) or an adjective (he is angry) or what. All we would know from "il est" is that we're talking about a grammatically masculine noun. In the same way you don't go around saying "He is" in English and expect everyone to know precisely what you mean.
So for adjectives we must match the noun in plurality and masc/fem. to do this when we have a plural masculine noun we add an S. when we have a feminine singular noun we add an E. (when its feminine and plural we add both s and e) There is an exception to this rule which is that if the adjective ends in a e and we need to make it feminine we do nothing. (in this case calmE) If it ends in an S and we need to make it masculine plural we do nothing. If we need to make it feminine and plural and it ends with an e we just add s.