To rescue Migel from the Prison in Second Son in Far Cry 6 sneak up to the west side of the prison and find Miguel inside the cell that has the security camera attached to it.
To find the key to his cell sneak around to the west side of the main building and climb the ladder to the top. Go along the rooftop to the east side of the main building and drop down to the next level.
Follow the corridor to the west and find the key inside the security office. Take the key back to the prison cell to rescue Miguel.