these two sites are very useful…

https://www.digitalocean.com/community/tutorials/how-to-create-a-self-signed-ssl-certificate-for-apache-in-ubuntu-18-04

https://www.digitalocean.com/community/tutorials/how-to-create-a-calibre-ebook-server-on-ubuntu-18-04

redirect incoming traffic to a service running at an internal port
–> use proxy option..

https://serverfault.com/questions/977894/redirecting-https-url-to-service-running-on-a-different-port

for example, if you have a ebook server running internal port 70 then,

setup proxy as ‘/ebook/’ ‘http://localhost:70″
and access this from outside by typing https://external_ip:port/ebook/

using this trick, you can link services as many as you want.