From Informed Comment
Trump caused a brouhaha by tweeting that Puerto Rican officials "only take from USA," as though they weren't part of the USA! Then his White House deputy press secretary Hogan Gidley called Puerto Rico "that country" during an interview on MSNBC Live with Hallie Jackson.
Trump seems to me to have some form of dementia, which would account for why he keeps saying strange things like that his father was born in a very nice place in Germany. Fred Drumpf was born in New York City in 1905 and later changed his name to Trump because of anti-immigrant (anti-German) sentiment during World War I.
So there are some ways that you can tell that Puerto Ricans are Americans.
1. Puerto Rico came to be part of the United States because on July 25, 1898, the US invaded the island, and then annexed it via the Treaty of Paris on April 11, 1899. (Sam Erman, "Meanings of Citizenship in the U.S. Empire: Puerto Rico, Isabel Gonzalez, and the Supreme Court, 1898 to 1905," Journal of American Ethnic History, Vol. 27, No. 4 (Summer, 2008), pp. 5-33.)
2. As early as 1904, the Supreme Court ruled in the case of Isabel Gonzales that Puerto Ricans could not be excluded by immigration authorities, inasmuch as they were not aliens (Gonzales v. Williams, 1904). This ruling was a baby step toward accepting that the island, having been annexed from Spain by the US congress, was no longer alien to the US. (Sam Erman).
3. The Jones Act of 1917 recognized Puerto Ricans as US citizens. Because Puerto Rico itself is a commonwealth rather than a state, Puerto Ricans residing in Puerto Rico cannot vote for president.
4. Charles R. Venator-Santiago writes, "In 1934, Congress introduced a territorial form of birthright citizenship permitting the children of Puerto Ricans born in the island to acquire U.S. citizenship at birth."
5. Since Puerto Ricans are full citizens, when they move to the mainland and reside in a state, they can vote for president, just like any other citizen of the US resident in that state. I have an educated guess for whom they are not voting in 2020.
6. Since 1952, even Puerto Ricans residing in Puerto Rico have had a representative in the House of Representatives, a non-voting "resident commissioner." Other countries don't typically have representatives in Congress, though there is a nagging doubt that Mitch McConnell actually views the Kremlin as his constituency.
7. Puerto Rico has introduced a bill to become a state by 2021. We don't usually let foreign countries become states.