Main Similarity and Difference
'They' and 'them' are both third-person plural personal pronouns. But one of them is a subject pronoun and the other is considered a determiner.
They: Subject Pronoun
'They' is a third-person subject pronoun. 'They' refers to a group of people, animals, or things.
Here, 'they' refers to a group of people.
Here, 'they' refers to a group of animals, for example a pride of lionesses.
The Singular They
Instead of using 'he' or 'she', we can use 'they' to refer to a person whose sex is not mentioned or is unknown. This is called the 'singular they'.
Many non-binary people prefer to be called by the pronoun 'they', rather than 'he' or 'she'.
'Everyone' does not specify if we're referring to men or women. Therefore, we use 'they' to include both of them equally.
Although, Elise is one person, but we used 'they' to be neutral about the gender of our subject.
We can use 'they' to refer to people in general, especially when we want to say a general idea or a famous saying.
You know what
Their: Possessive Determiner
'Their' is the possessive form of 'they'. We use 'their' to express that something belongs or relates to the group of people, animals, or things.
Kevin and Paula are going to name
We can also use 'their' instead of 'his' or 'her' to show that something belongs or relates to a person without specifically and explicitly saying whether that person is a man or a woman.
Every boxer should compete according to
Everyone should sign
In many countries, they still eat all