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.")
Thank you, JeffreyD for this fantastic and clear explanation. I found it to be most helpful.
The way i understand it 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 "femme" and then "elle" 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 "Elles sont riches"! (no secondary gender reveal)
I found this article that helped... https://www.thoughtco.com/french-expressions-cest-vs-il-est-4083779
I'm still unclear how to use adjectives in this case.
This thought may be of use to Irish speakers. There are some parallels with the Irish copula is (a defective verb cum complementising particle) and the fully conjugable verb tá. One helps to identify something, the other to describe its characteristics.
"Is Bean í" (She is a woman/It's a woman) C'est une femme
"agus" (and) et
"tá sí saibhir" (she is rich) elle est riche.
Not a particularly accurate comparison, but it helps me get the C'est/Il est distinction right most of the time.