Which matcher does Caddy pick when there is a conflict?

In my understanding and experience reverse_proxy takes precedence because directives have a default implicit order: https://caddyserver.com/docs/caddyfile/directives#directive-order .

To force a different order you have at least 3 options:

1. use a route:

    route /api/* {
      file_server
      reverse_proxy localhost:9000
    }

2. change the default order with the order global option
3. use handle with file_server before reverse_proxy

From my understanding and experience, more specific is taken into consideration over more generic.

For instance in your case, anything matching /API/ should be forwarded to node, and anything else forwarded to the normal webserver.

At any rate, as long as /api/ isn't an actual folder on the website, requests hitting the website won't matter

Also see: https://caddyserver.com/docs/caddyfile/directives/root