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If you're not a software developer, then there's nothing to download. It's not a single app or feature, it's a process that makes it easy/easier for developers to bring apps originally written for iPhone/iPad over to Mac. It's up to the app developer to do this, an end user can't just take a random iOS app and make it run on Mac.

If you are a developer... Project Catalyst is supported in Xcode 11. Install Xcode 11, and refer to the resources at https://developer.apple.com/mac-catalyst/
 
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I'm under the impression that's a component of Xcode, intended for developers.

It's not a standalone application for end users.
 
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