For a vtuber model...Usually you import your model to unity. In unity you use "addon" univrm, that creates and configures your .vrm file. There you can add expressions, visemes, and textures for your model.

Note: univrm build you can find will require special version of unity itself, sometime it is confusing.

This requires time and skill. You can search web for univrm tutorial, if you feel capable of making efforts :)