As far as we can see from the limited leaks to date orchestrated by Schiff and the Dems, their primary witnesses are career, unelected bureaucrats complaining about Trump's execution of his foreign policy. But this is one of the things the 2016 election was about ("drain the swamp').
And besides the President's power over foreign affairs is plenary or exclusive. When the framers sought to limit that power, they did (for ex, a declaration of war)
"[t]he President has primary responsibility for the conduct of the foreign affairs of the United States," and although Congress has some specific powers that "concern or bear upon foreign affairs[,] .. the presidency is the institution on which the Constitution places the duty to look to the Republic's interests in the international arena."
Jefferson Powell, The President's Authority over Foreign Affairs: An Executive Branch Perspective, 67 GEO. WASH. L. REV. 527, 535 (1999).
If you really like this topic, here is the full DOJ paper on the plenary power under the Constitution of the President over foreign affairs. I think this one was written in contemplation of the US entering WWII. https://www.justice.gov/file/20661/download
And, FWIW, the SCOTUS is still singing the same song even today --
“[j]udicial inquiry into the national-security realm raises concerns for the separation of powers” by intruding on the President’s constitutional responsibilities in the area of foreign affairs. Ziglar v. Abbasi..."
And even from Kennedy (of all people) in the same recent case --
.... substantial deference that is and must be accorded to the Executive in the conduct of foreign affairs .... And even if further proceedings are permitted, it would be necessary to determine that any discovery and other preliminary matters would not themselves intrude on the foreign affairs power of the Executive.
See https://www.supremecourt.gov/opinions/17pdf/17-965_h315.pdf