How to get memoji app

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Use Memoji on your iPhone or iPad Pro - Apple Support

According to:

Use Memoji in Messages on iPhone - Apple Support

On models with a TrueDepth camera, you can send animated Memoji messages that record your voice and mirror your facial expressions.

Memojis are intended and used to alter your physical appearance into the apple cartoon schematic. It does not alter your voice although on the Message app you can record and send animated Memojis with your facial expressions and voice recordings.