iPadOS 14.5.1, go to Settings —> General —> Software Update on your iPhone and iPad. Per Apple’s release notes: This update fixes an issue with App Tracking Transparency where some users who previously disabled Allow Apps to Request to Track in Settings may not receive prompts from apps after re-enabling it. This update also provides important security updates and is recommended for all users.
watchOS 7.4.1 also contains security improvements. It can be downloaded through the dedicated Apple Watch app on the iPhone by going to General —> Software Update. To install the new software, the Apple Watch needs to have at least 50% battery, needs to be placed on a charger, and it needs to be in range of the iPhone.
To access macOS Big Sur 11.3.1, go to System Preferences —> Software Update on your Mac. The update contains security improvements.