Shipments of the magnets, essential for assembling everything from cars and drones to robots and missiles, have been halted at many Chinese ports while the Chinese government drafts a new regulatory system. Once in place, the new system could permanently prevent supplies from reaching certain companies, including American military contractors.
Trump wants to devalue US currency. He sees US dollar supremacy as why the US is not competitive in the export market and part of the reason why manufacturing has left the country.
Most countries take the US dollars they make on exports and stockpile them in US treasuries which keeps the dollars value high relative to other currencies.
Essentially he sees the high value of the US dollar and trade deficit as bubbles that will eventually burst and collapse the US economy. Which is an issue he thinks he can fix/prevent with tariffs.