After Moses had removed Aaron’s garments and put them on his son Eleazar, Aaron died there on top of the mountain. Then Moses and Eleazar came down from the mountain.
The chapter began with the funeral of Miriam, and it ends with the funeral of her brother Aaron. When death comes into a family, it often strikes double. Israel had not improved the former affliction they were under, by the death of the prophetess, and therefore, soon after, God took away their priest, to try if they would lay that to heart.
Commenting on Numbers 20:22-29
And when all the congregation saw that Aaron was dead,.... Not that they saw his dead body, but they perceived by the relation of Moses, and by various circumstances, as not seeing Aaron come down, whom they saw go up, and seeing Eleazar with Aaron's garments on him, and perhaps by tokens of mourning in Moses and Eleazar; so the Targums of Jonathan and Jerusalem...
Aaron died there in the top of the mount--(See on Deu 10:6). A tomb has been erected upon or close by the spot where he was buried.