compared to Max, I would be less emphatic about never using bad grammar. Is there never a time to use bad grammar? I do not use it, when I am at my best. I do not indorse it, and I do not agree it is a good choice. Yet I can admire creative speech and am willing to recognize that many people that use "bad" grammar are doing it by choice. To make a point? To be a part of a culture? To further differentiate themselves from other people - such as myself? I can see valid reasons for it - even if the outcome will often be a negative monetary outcome for the speaker (if the habit follows them to work.) A native speaking person that says "Where them at?" surly could do better if they chose. I am playing the devils advocate here - but still - I say, never use never. ;)

It is bad grammar. You would never say "Where are them at" under any circumstances. You might hear "Where are they at?", which is still bad grammar. Where are they = At what location are they So "Where are they at" = "At what location are they at" (wrong) "Where are they?" and "where are the girls?" are both correct. Sometimes, you will hear "Where are you at?" in reference to a project or task. "Where are you at?" = "Where are you at in the project?" = "How much progress have you made in the project?" It is still wrong! "Where are you in the project?" is correct.

'Where them at'? and 'where they be'? are used instead of 'where are they?' They can be heard in African American films and some African Americans do speak like this. Their language is officially recognized and it is called Ebonics. The second one is just slang for 'where are the girls?' You will hear lots of slang in the 'real world' It's not correct but it's here to stay. Just don't use slang when you are writing your Thesis or applying at University.

that doesnt exist

I would just recommend not using that phrase！