Learn English – “Them are” or “They are”

They're not quite the same, and one of them is incorrect.

They: the things we were referring to earlier. For example,

My aunt and uncle live in Brixton. They have a big house.

So you might say, "These things are (whatever). They are called living things."

Those: the plural of that. We use it when referring to specific things that are farther away or more removed from our observation or discussion. For example,

These trees here are apple trees. The ones down in the valley? Those are pear trees.

So you might use "those" like, "These are rocks. They are not living things. But there are also animals. Those are called living things."

Them: Incorrect in this case. It's the object version of the pronoun they. You would use it like

My aunt and uncle live in Brixton. I went to visit them.

You can't say them are, because you need a subject there, not an object.

References:
These, Those, and They
Personal Pronouns