What's the difference between 你在哪儿? and 你在哪里?

they mean the same but
你在哪儿 is very verbal. beijing people likes to speak with a lot of 儿

both are the same,no difference.哪里和哪儿，这里和这儿，那里和那儿

The difference between "你在哪儿?" (nǐ zài nǎ'ér?) and "你在哪里?" (nǐ zài nǎlǐ?) is subtle and both are used to ask "Where are you?" in Chinese. However, "你在哪儿?" is more informal and is often used in spoken language, while "你在哪里?" is more formal and is used in written language or in more polite conversation.

No difference, 你在哪儿 is more oral language.

你在哪儿? and 你在哪里? both mean "Where are you?" in Chinese. They are both grammatically correct and commonly used, but 你在哪里? is more formal and polite, while 你在哪儿? is more casual.

In general, when speaking with someone you are close to, such as a friend or family member, you may use 你在哪儿? but in a more formal setting, such as at work or when speaking to someone older or in a position of authority, you may use 你在哪里? as a more respectful form of address.

Both of them has same meaning which is "Where are you? "
你在哪儿 is used by people from north of China, it is also called Beijing accent.
你去那里 is used by people from south of China.