Suppose I want to use a combination of NGinX (probably another since it doesn't proxy HTTP/2 requests) and Hypercorn. As both can handle SSL certificate files, I wonder who is the best suited to do this for an HTTPS request. It is important to me that Hypercorn could listen to 443 port and I'm not sure it can do that without specifying certfile and keyfile parameters.
Well, that depend what you want to do.
The simpliest solution is to configure both to use SSL.
Nginx will receive the request, decipher it, process it, send it to Hypercom on port 443 as an HTTPS Client. Hypercom will get the request as any normal HTTPS client.
Nginx support by default proxying request to an HTTPS upstream so that's the best solution I think. However, you might need to play with setting http-header for hypercom to correctly understand who's the client by playing with X-Forwarded-For, X-Forwarded-Host and any headers that might be needed by Hypercom.