The "official" review score is an aggregation of people who regularly review movies across a wide variety of publications from mainstream media outlets (NY Times, BBC, The Guardian, Rolling Stone, etc.) to smaller pop culture websites (Nerdist, io9, Looper, etc.) and internet personalities like John Campea and a bunch of prolific bloggers. There's over 3000 active reviewers who contributed to these ratings, though only up to several hundred of them may review any particular film. The "Top Critic" score filters out all the internet personalities and niche websites, aggregating reviews from only the more renowned mainstream media outlets.
Each individual review is broken down to a fresh or rotten rating based on the score given by the reviewer. Anything > 50% is considered fresh, 50% and under is rotten. The aggregate review score is calculated using the formula (fresh reviews / total reviews) * 100.
I wonder how much being a critically acclaimed director actually matters on these MCU films that are directed by committee. Lucrecia Martel turned down Black Widow after being told they already had other people who would be responsible for directing all of the action scenes.