If you want to have multiple Solr indexes using Search API, you need to have a core for each index instance. For my local development stack, I use DDEV.

The JSON API module is becoming wildly popular for in Drupal 8 as an out of the box way to provide an API server. Why? Because it implements the {json:api} specification. It’s still a RESTful interface, but the specification just helps bring an open standard for how data should be represented and requests should be constructed. The JSON API module exposes collection routes, which allows retrieving multiple resources in a single request.