Car dies while driving

Probably not the starter relay since that wouldn’t kill the engine. First step would be to see if there’s anything specific that’s happening when the engine dies. For example, is it only after reaching a certain temperature? Or only when turning/going straight/hitting a bump? If it only starts again after cooling down, that indicates it’s a cooling system issue. There might be an overheating problem or there might be an ECU issue that’s misinterpreting temp sensor or other engine parameters. Unless the engine dies at a very specific time, every time, it could be difficult to diagnose without ECU/BCU/engine code reading tools.