Nitpick: the boosters were reused by parachute splashdown, but it was also really expensive due to (among other things) the corrosiveness of salt water. The External Tank was in fact expended.
Each SRB consisted of 11 cylindrical steel sections. After flight and recovery, these were dismantled, refilled with solid rocket fuel as appropriate, and then reconnected to form a new SRB with all of the O-rings, etc.
So also a complete rebuild, with reuse occurring at the segment level, not of the whole booster assembly.
So also a complete rebuild, with reuse occurring at the segment level, not of the whole booster assembly.