SOLAS is about the ship — construction, equipment, fire safety, life-saving appliances, radio, navigation safety, and safe operation systems.
STCW is about the people — minimum training, certification, and watchkeeping standards for seafarers.
They work together: a SOLAS-compliant ship still needs STCW-qualified crew, and STCW training (e.g. BST, fire-fighting, survival) teaches how to use the safety systems that SOLAS requires on board.