The reality is that they get sent back to their home country, after extracting whatever knowledge exists in US-side operations.
But you need to keep one important thing in mind - almost all the H-1Bs are completely “average” engineers and scientists who replace completely “average” American engineers and scientists.
Very few H-1Bs go home and start high tech companies that seriously compete with USA companies. Just like very few “average” Americans start high tech companies that seriously compete with other well established American companies.