General Discussion

Magnets can be found in nature, or manufactured. The Adirondack Mountain of New York State contain deposits of lodestones, a natural magnet. Lodestones are also found in many places around the world.
The process of using Powder Metallurgy to create powerful magnets does us rare earth elements, but these elements are found all over the planet, including the United States, although China has the largest deposits in the world.
So, Trump is partly correct, but the need for rare earth elements from China is probably related to quantity and/or price. It's not necessary to get them from China.
For instance, Ukraine has significant deposits of rare earth elements. Remember Trump was trying to make a deal to get them from Ukraine in return for military/financial aid.
As usual, Trump fixates on a thing and keeps harping on it.