Step 1: Take note of all the keyframe times in your animation.

Step 2: Follow this guide on importing and preparing your model in Unity, but don't export yet: http://help.plattar.com/assets-and-content/advanced-topics/exporting-complex-animations
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Step 3: Set up your multiple animations in the model import window. You will be able to preview each animation to make sure the timing is correct.
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Step 4: Select your model in your scene. It should have the animations listed under the Animation component:

Step 5: Export To Plattar

Apple USDZ file format has limitations on animation and rigging. Please refer to File Formats USDZ Section for more details on this limitation and potential workaround solution.