Request multiplexing over a single TCP connection is the most advanced feature of HTTP/2 protocol over HTTP/1.1.
The image below perfectly shows this ..
See below some of the good resources to learn the difference between HTTP/1.1 and newer HTTP/2.
HTTP/2 is still only have 1/3rd of market share so time to upgrade.
btw I am also lagging behind in upgrading my legacy HTTP/1.1 systems to HTTP/2 🙂