Each is a separate piece, it's not either-or. A "static site" means render.com would be serving up just static files to users, but those can include client-side JS files. Now, those files wouldn't have a server to talk to (unless they're talking to third-party APIs), but you could then set up a service and/or database alongside them. Pricing is different for each of those, but it looks like base-tier web services are also free.

Hope that helps