I study how third-party firms respond when a platform owner enters its own marketplace, analyzing Apple's entries into App Store submarkets from 2016-2021. Using text embeddings to define markets and a staggered difference-indifferences design, I find that Apple's entry deters new competitors and shifts incumbents' monetization strategies, but effects vary widely: many markets show no meaningful response, while others move in opposing directions across a host of monetization and quality outcomes. Responses depend on how Apple enters and apps' competitive proximity to Apple. This heterogeneity suggests targeted oversight rather than categorical restrictions on platform-owner entry.