Why it's similar
X.app is an all-in-one extension app across iOS, iMessage, watchOS, and visionOS with an in-app browser and picture-in-picture, overlapping X's content-consumption side. Its 4.65 rating across 4,885 reviews shows decent adoption.
How it differs
X.app is a utility-and-browser tool rather than a social network like X, so it doesn't replicate X's posting and timeline. It shares the name but serves a different purpose, with in-app purchases, making it a companion utility rather than an X replacement.
