"Where's your home?"
哪: (indicating a question, such as what/which/how) (in the case of asking for a location, it's more like "what" or "which"
哪儿: (similar to or the same as 哪, with the Beijing-style (and certain-other-dialect-style) final consonant 儿 - see more about 儿 in the bottom of this post)
里 in 哪里: a certain spatial area/range
哪里: what/which spatial area/range, what/which place (which has the effect of "where")
在哪: at/in/etc. what/which [place] (think of it as 在哪里 with 里 omitted)
在哪儿: (similar to or the same as 在哪, with the final consonant 儿)
在哪里: at/in/etc. what/which place
When used as a syllable and pronounced as ér, it means "child," "juniors (in terms of age, experience, status, rank, etc.)," "young male," etc.
When used as the final consonant of the previous character and pronounced as r (pinyin), it can add diminutiveness to the word right before it, although sometimes it's added simply out of habit. In this case, the character right before 儿, which got an extra final consonant r, is said to have 儿化音 (r-ized sound). Note that the 儿 in 儿化音 is pronounced as ér.
Doing this question using a keyboard and a Pinyin input, I really wish I knew how to type "哪儿". When I type "nar" the first entries on my Pinyin keyboard layout (Intelligent Pinyin) are 那人，纳入，⬇ [Yes! A downward arrow]，那日，and scrolling five pages still don't see "哪儿". Any suggestions?
I worked it out with thanks to @GryTea from their answer on this page (do take a look!). For anyone else stuck tying answers using a Pinyin keyboard, keep reading.
The 儿 (as a separate character) would be pronounced "er", but when placed after another character it becomes just "r".
To type 哪儿 on a Pinyin keyboard, you therefore type "naer", not "nar" (even though it is romanized and pronounced the later way).