It's also possible that these Doctors are the elder versions of the Doctor that have popped up, and possibly even explain the appearance of Tom Baker's The Curator in the 50th-anniversary special, "The Day of the Doctor."

Source: https://comicbook.com/tv-shows/news/doctor-who-special-3-the-giggle-watch-bigeneration/

The success of Baker's efforts highlights the potential of practical effects and the importance of time and effort in achieving stunning visuals.

Source: https://screenrant.com/oppenheimer-movie-nuclear-explosion-effects-recreation/