https://www.cgcircuit.com/tutorial/auto-blendshape-transfer-different-topo

Many Facial Rigging Vendors and Studios charge thousands of dollars to create a high level facial rigs either from scratch or to transfer multi-blendshape expression rigs to another character. We are going to change that!  In this training series you will learn the secret to transferring blendshapes across different topology characters. We’ll cover migration methods similar to those used at the top studios behind Unreal MetaHuman, Ziva Dynamics Face Trainer, Snappers Facial Rig, PolyWink Transfer, and more. Too many Studios fall into a single topology asset pipeline limiting art direction and character flexibility. While similar topology presents advantages where we can reuse items like models, textures/shaders, and skinweights; this surrounds us in a box that’s hard to escape when one of our characters has a unique facial element like a horns or asymmetry.  What if there was a way to create your Blendshapes for one character and then easily transfer to multiple characters and overcome that limitation?  We would be able to focus on building a strong base shape library. Imagine the difference between creating five characters each with 50 blendshapes manually versus creating 1 character and automatically transferring one set of 50 blendshapes to our remaining four characters.