...your insurance? policy covers only $100,000 per person, where does the money come from to make up the difference?

If you are found liable for the injuries of the other person, then you are liable for that amount. If they get a judgment against you for $1.5M, then they can go after your house, your business, your personal property, 401(k), etc. If you don't have the money or don't own property, you are considered "judgment proof." Some people might consider going into bankruptcy after taking such a hit.