The 's is added to a noun to show belonging. This is my house. This is his house. This is the doctor's house. This is Julie's house. When the noun ends with "s" we just add an apostrophe after the "s". Those are the babies' beds. And these are our dogs' beds. These two rules have nothing to do with singular or plural, only with the "s" at the end of the word. Are these the children's bicycles? When a person's name ends with an "s", then you can double up with 's. Miles's car is over there.

s' is used for avoiding s's the students's books ---> the students' book students's is wrong form students' is correct form

s' is used for plural forms. For example, "Friend of my brothers lives in Russia". The word "brothers" is plural. So we may say - "My brothers' friend lives in Russia".