yes..Stephen quotes an Old Test Scripture, and that in doing so he is making the claim that this person Jesus was the Christ talked about in the Old Test verse..
But you may ask..."
Where did Stephen get this idea for being on the "right hand" ?
I believe the answer is that he got the idea to say this from listening to
Peter on the
Day of Pentecost.
If you check out the story about that event, you will see
https://www.biblegateway.com/p***age/?search=Acts+2
"Exalted to the right hand of God, he has received from the Father the promised Holy Spirit and has poured out what you now see and hear."
I believe what we read in the last words of Stephen is the final teaching of him to the Jews who were killing him at the time.
He quotes the teaching that he likely heard from Peter, who in the very first sermon of the Christian church does make use of these words about the
"right hand of God" as it actually is taken out of the Old test verse written by King David.
So Stephen is quoting a sermon by Peter, and Peter was quoting a verse likely written by David.
The expression that David made use of was a common idea that being that the "
right hand" was a place of honor due to it being not the "left hand" as the left hand was the hand that everyone at that time cleaned their **** with.