In the "old days" -- perhaps eight years ago -- the trained observers made the defensive assessments on each play.
Today those calls are made by Statcast and similar devices that precisely track, among other things, the angle and velocity of the ball as well as the position of the player.
For better or worse, technology has taken over the game. I recommend the new book The MVP Machine: How Baseball's New Nonconformists Are Using Data to Build Better Players by Ben Lindbergh and Travis Sawchik:
https://www.theatlantic.com/magazine/archive/2019/07/building-the-next-babe-ruth/590641/