Apple Watch complications
How do I add a third party app to my list of complications on my Apple Watch ?
Apple Watch Series 5, watchOS 6
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Use iPhone -> Watch app
Find app via the Watch app that have Apple Watch components
Use the Watch app to install them on your Apple Watch
If the complication position on the Apple watch can support the 3rd party complication, then it will appear in the list of complications you can set for that location. Not all complications work in all locations and with all watch faces.