Adam Clarke on Exodus 17:9

Moses said unto Joshua - This is the first place in which Joshua the son of Nun is mentioned: the illustrious part which he took in Jewish affairs, till the settlement of his countrymen in the promised land, is well known. He was captain-general of the Hebrews under Moses; and on this great man's death he became his successor in the government. Joshua was at first called Hoshea, Num 13:16, and afterwards called Joshua by Moses. Both in the Septuagint and Greek Testament he is called Jesus: the name signifies Savior; and he is allowed to have been a very expressive type of our blessed Lord. He fought with and conquered the enemies of his people, brought them into the promised land, and divided it to them by lot. The parallel between him and the Savior of the world is too evident to require pointing out.

Top of the hill - Probably some part of Horeb or Sinai, to which they were then near.