You need to use different suffixes depending on the whether the subject is referred to in first, second or third person. That's the whole basis of the verb conjugations. The actual order would be:
Singular -- Plural
Je mange -- Nous mangeons
Tu manges -- Vous mangez
Il/elle mange -- ils/elles mangent
In case you're not directly referring to the subject as I or you (je, tu, or vous), the subject is considered third-person. So "the man" (le homme) would be third person, and you'd have to use the mange (s) or mangent (pl) form.
Though all the conjugations do sound almost exactly identical. If you're hearing them, you need to decide the conjugation based on the context.
"Elle mange des frites" means that she is eating more than one bean. In English, there is no plural form for "a/one", but in French, the plural of "un" or "une" is "des". So, "des frites" is the plural form of "une frite".
"Beans and carrots" can translate to "les haricots et les carottes" or "des haricots et des carottes", as long as there is no context to specify whether the English fragment is the plural of "a bean and a carrot" or "beans and carrots in general".
Yes, elle is she (singular). Elles is they, referring to more than one female.
"Des" is the plural indefinite article that English does not have.
"Des" is the plural of "un/une" and it is required with the meaning of "more than one". Sometimes, you can translate "des" to "some", but more often than not "some + plural noun" will not back translate to "des + plural noun".