How does server return JWT token to the client?

there is no standard for how to return JWT token to the client, however, check this URL, it answers your question

https://github.com/dwyl/hapi-auth-jwt2/issues/82#issuecomment-129873082

putting the JWT token in the Authorization header gives us flexibility to send an actual response in a web application. For a REST-only App/API you are free to send the JWT as the response body or a cookie. What matters is how the client stores the JWT and sends it back to the Server, which is done in the Authorization header (or Cookie or URL Token if you prefer) 👍

As for this existing in the “wild”, I have not seen an example of the server sending an Authorisation header to the client, but there is nothing in the spec to suggest this is an anti-pattern.
see: http://self-issued.info/docs/draft-ietf-oauth-v2-bearer.html

If you want to stick to the guidelines you would do follow this example: http://self-issued.info/docs/draft-ietf-oauth-v2-bearer.html#ExAccTokResp