When curl is built, it gets told to use a specific TLS library. That TLS library is the engine that provides curl with the powers to speak TLS over the wire. We often refer to them as different "backends" as they can be seen as different pluggable pieces into the curl machine. curl can be built to be able to use one or more of these backends.
Sometimes features and behaviors differ slightly when curl is built with different TLS backends, but the developers work hard on making those differences as small and unnoticeable as possible.
Showing the curl version information with curl --version includes the TLS library and version in the first line of output.
When curl is built with multiple TLS backends, it can be told which one to use each time it is started. It is always built to use a specific one by default unless one is asked for.
If you invoke curl --version for a curl with multiple backends it mentions
MultiSSL as a feature in the last line. The first line includes all the
supported TLS backends with the non-default ones within parentheses.
To set a specific one to get used, set the environment variable
CURL_SSL_BACKEND to its name.