'You're' is a contraction of 'you are' it is a 100% rule and it has no other usage. If you cannot use 'you are' instead of 'you're' in a sentence it means the sentence is incorrect.
The word 'your' is usually used before a noun or a pronoun to show that something belongs to someone or is related to someone. Like in the following sentences:
You're and your are not interchangeable and have absolutely different meanings.
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