how were copies of vinyl made in "third-party" countries

Classical LPs pressed in the US under the Angel label were typically dreadful compared to UK equivalents. ECM LPs of course made in Germany are always technically superb. So I think you go on a case by case basis which is why I questioned the “third party” description.

To add a bit to the cacophony, in the 60s, 70s, and 80s, where you sometimes had the same performance available on Decca, London, and/or Angel labels, one always knew to seek out the British pressing over the US pressing. Same went for EMI vs Capitol.

Why do you use the term “third party” with respect to Holland, Russia, Hungary, Yugoslavia? All those countries had great orchestras iand first rate recording and record manufacturing facilities in the 80s and beyond.