> I might understand this incorrectly, but doesn't it also mean that American businesses have a greater market to sell to?

"American" companies aren't always entirely owned by Americans. Sometimes they get purchased outright by foreign conglomerates or shareholders (eg. Anheuser-Busch) or take on foreign investment (eg. SoftBank). Once they're public, lots of entities can own the stock.

Labor doesn't have to be based in the US. Wages go overseas all the time. Your iPhone is manufactured abroad. Amazon hires lots of non-American programmers.