What would happen to the U. S. economy if all or most manufactures here made 80-85% of their own parts like SpaceX and Tesla?

The U.S economy won't fall, but it will have some problems.

Explanation:

Now,

If all  manufactures start making their own parts, the U.S Government wont have any customers to buy the resources needed to make a part. These Manufacturers will have to make mines and factories of their own to do this. It is unlikely that manufacturers will make factories or mines to make their own parts because it is going to be a huge project and its going to cost a lot of money which will be a lose-lose situation because both the U.S and the manufacturers are not going to benefit.

the correct answer is economics is a game without winners or losers.

unlike sports or games economic is a game without losers or winner rather it is a game where everyone wins. parties of business doing businesses exchange money with each other and hence exchange is done on the basis of goods. one party takes the goods and other sales it so there is winner on both sides. each party values more highly the good it gets than the good it gives up. both sides exchange only because both intend to increase their value. so there is no concept of lose or win.

15 pounds of apples and 6 cups of flour