“you all.” The you is plural.
“ascending and descending on.” This is the literal reading of the Greek text, and is designed to vividly call to mind the record in Genesis 28:12 where Jacob saw a great staircase going to heaven with angels going up and down on it (it was not a “ladder” as we know it today, even though many versions use “ladder.” It was a staircase, as per the HCSB, NAB, NET, NIV, etc.).
Although the Greek uses “on” (the preposition epi), the Apostles would have understood what he meant, that it was “on” him, idiomatically meaning due to who he was, that the angels were ascending from, and descending to, him. As the Son of God, Jesus was now the focal point on earth for the angels who were coming from and going back to God.