Hello. Interesting question. "as" is used to introduce two events happening at the same time. In your example the Events are the audience following and Greg cooking. "That" is used when talking about objects or groups. "The team that" or " the ball that" So "who" is more appropriate for the sen sentence. However as is more suitable because you speak of two events that should happen simultaneously. And "that/who" in this example works more as an introduction of what/who Greg is and what he does. I hope this helps. You are more than welcome to ask more questions. :)

1. "Follow our favorite foodie, Greg Romero, as he tries the delicacies of the world like Peru, Morocco, and Japan." In this sentence, "as" is used to indicate a simultaneous action or circumstance. It implies that Greg Romero is actively engaging in the act of trying different foods from various countries. By using "as," the sentence suggests a direct connection between following Greg Romero and witnessing his culinary adventures. It emphasizes the journey or process of Greg trying different foods. 2. "Follow our favorite foodie, Greg Romero, who tries the delicacies of the world like Peru, Morocco, and Japan." Here, "that" or "who" introduces a relative clause that provides additional information about Greg Romero. The clause "who tries the delicacies of the world like Peru, Morocco, and Japan" describes Greg Romero as a person who has the characteristic of trying various foods from different countries. Unlike "as," which emphasizes the simultaneous action, "that" or "who" emphasizes Greg's identity or trait as a foodie who explores international cuisine. This construction suggests that following Greg Romero is valuable because of his consistent exploration of global culinary delights. ==To put it simply, "As" emphasizes the action of trying foods, while "that" or "who" emphasizes Greg's identity as someone who regularly engages in culinary exploration.