Why is 我的朋友今年住在美国 translated to both "my friend [is living in / lived in] America this year"?
Chinese language doesn't have tense. To indicate event happened in the past, you need to add time reference in the context or verb particle that indicate past verb

Example:

我的朋友今年在美国(住过) = My friend (had lived) in America this year

过 is a verb particle that indicates 'experienced' aspect of the verb

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我的朋友今年(曾住)在美国 = My friend (had lived) in America this year

曾 is an adverb that indicates 'once'; formerly

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You can use either or both in the same sentence:

我的朋友今年(曾)在美国住(过)