Learn French: Le garçon chez qui nous mangeons est attirant. - The boy at whose house we are eating is attractive.

In French, dont means 'whose' when showing possession, like 'the boy whose dog is cute'. However, 'at whose house' is really just the preposition chez (at the home of) plus a person. For a preposition followed by a person, French uses the preposition + qui. So, 'at whose house' becomes chez qui (literally: at the home of whom).

Does qui always mean 'who' or 'that'?

Usually yes, but when qui comes right after a preposition (like chez, avec, pour), it means 'whom'. In this sentence, chez qui translates literally to 'at the place of whom'.

Could I use où (where) instead of chez qui since we are talking about a location?

Not in this sentence. The relative pronoun has to match the noun it follows. Because it directly follows Le garçon (a person), you must use a preposition + qui. You would only use où if the preceding noun was a place, such as la maison où nous mangeons (the house where we are eating).

I see that attirant ends in '-ant'. Is it acting as a verb or an adjective here?