What make distro like Fedora secure/safe?

This is a meta question. The code base of modern Linux distro is Large. For example, how to make sure that distro is not spying use by any level of parts of any open source project in base of whole distro. In other words, What make distro like Fedora secure/safe form regular user point view e.g how to check it this base element safe/secure?

From Ask Fedora to The Water Cooler

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