How we handle the conversion of the class based to the functional based component.

This sounds like a general React question. I’m not familiar with React, so best to ask on Stackoverflow I googled for “algolia dropdown refinementlist” and found this guide on how to build what you’re looking for: https://www.algolia.com/doc/guides/building-search-ui/ui-and-ux-patterns/facet-dropdown/js/