Learn Dutch: Nee, hij is niet thuis. - No, he is not at home.

Huis simply means "house" (the physical building). Thuis is a separate word that means "at home". You use thuis when talking about someone being at their own place, just like in English we say "at home" instead of "at house".

What would the yes/no question for this answer look like?

The question would be: Is hij thuis? (Is he at home?). Notice how the verb (is) moves to the very beginning of the sentence to form the yes/no question, which is exactly the rule you are practicing in this lesson!

Is the "th" in thuis pronounced like it is in English?

No, the Dutch "th" is always pronounced exactly like a regular "t". It does not have the "th" sound you hear in the English word "think". The word thuis starts with a crisp "t" sound.

Why does niet come right before thuis?

In Dutch, the word niet (not) is often placed directly in front of the specific word or phrase it is negating. Since the sentence is specifically negating the location (thuis), niet goes right before it to mean "not at home".