NOT an expert, but since no one else is answering your question:
1) Many US doctors take their cues from NIH and FDA. When the FDA gave Remdesivir an Emergency Use Authorization, it was applied in 100s of hospitals because it was something and there wasn't anything else. If doctors had looked more closely at the research, they might have NOT gone with the $3000/treatment drug because the trial results were so weak.
2) Based on NYC MD/PhD Daniel Griffin's comments on the This Week in Virology podcast, I gather that hospitals and health care systems have people assigned to the task of monitoring research and government announcements. They regularly revise their standard treatment and convey changes in seminars or by email.