Here "the dead" is a nominal adjective that means all dead people together in the underworld. To be raised from among them means to become alive again. Alternate translation: "when he brought him back to life" or " when he caused him to be alive again" (See: [[rc://en/ta/man/translate/figs-nominaladj]])
The person who sits "at the right hand" of a king sits at his right side and rules with all the authority of the king at whose right hand or side he sits. This is a metonym of location that represents the authority that the person in that location possesses. Alternate translation: "gave him all authority to rule from heaven" (See: [[rc://en/ta/man/translate/figs-metonymy]])
To sit at the "right hand of God" is a symbolic action of receiving great honor and authority from God. Alternate translation: "seated him in the place of honor and authority beside him" (See: [[rc://en/ta/man/translate:translate_symaction]])