Tom plays basketball daily unlike Sam, who is a very busy man
Is the above sentence correct?
So 'who' always relates to 'Sam' or the nearest noun (& not Tom)?
The question pronoun 'who' is placed after Sam and that too defined by the comma in this sentence. Indeed, it talks about Sam and not Tom.
In this sentence, it's clearly defined that unlike Sam...who.... so it leaves no ambiguity in understanding the sentence. That said, yes, you are right.