You can’t, there’s a filter to convert everything to jpeg on upload or everything to webp, but short of installing a 3rd party service to figure it out on the fly there is no fallback solution that serves webp to some and jpeg to others
I thought wordpress would manage this but it does not.
Correct, WP does not handle this, and there is no canonical solution for this. If you need to support browsers that do not support webp, don’t use webp, convert everything to JPEG via filters on upload, or use a 3rd party CDN service that will optimise images on the fly.