What's the plural of family?

What's the plural of family?

families
Change y into I and add es

b. For plural nouns ending with - s, add the possessive sign 'at the end
For example: her friends' pictures
Why do Keats' Add 'directly without's'? Is the name followed by's' the same way

No, because names are not plural, the possessive case of nouns in English can be roughly divided into three types. 1. After singular and plural nouns not ending with s, such as the teacher's name, children's day. 2. After plural nouns ending with s, such as the teachers' office. 3, Generally, we don't use's' expression. For example, the color of the box is expressed as: the color of the box, not the box's color

On the possessive case of nouns: when nouns are plural or words ending in - s or - es, add 'right' after them, then why is' Dumas's' in my book
Dumas is Dumas
This book was sold faster than any other Dumas's
But nouns ending with s or ES only add "'"
Isn't Dumas just a noun ending in S

Dumas is a whole, not a plural

Is possessive s the same as plural and singular s
If not, please give me a detailed distinction,

It seems different
The s of the possessive case of a noun seems to be a clear consonant
The plural noun and the singular third person s will pronounce consonants or voiced consonants according to the difference between the end of the word