The HTTP versions 2 and 3 offer "multiplexing". Using this protocol feature, an HTTP client can do several concurrent transfers to a server over the same single connection. This feature does not exist in earlier versions of the HTTP protocol. In earlier HTTP versions, the client would either have to create multiple connections or do the transfers in a serial manner, one after the other.
libcurl supports HTTP multiplexing for both HTTP/2 and HTTP/3.
Make sure you do multiple transfers using the multi interface to a server that supports HTTP multiplexing. libcurl can only multiplex transfers when the same hostname is used for subsequent transfers.
For all practical purposes and API behaviors, an application does not have to care about if multiplexing is done or not.
libcurl enables multiplexing by default, but if you start multiple transfers at the same time they prioritize short-term speed so they might then open new connections rather than waiting for a connection to get created by another transfer to be able to multiplex over. To tell libcurl to prioritize multiplexing, set the CURLOPT_PIPEWAIT option for the transfer with curl_easy_setopt().
With curl_multi_setopt()'s option CURLMOPT_PIPELINING, you can disable multiplexing for a specific multi handle.