Curl is in everything. And you can tell, if you find the license. In my 2014 Chevy Cruze, if you look in the owner's manual, there is the curl license, in all its glory. I've used it many times myself, and it's always proven itself to be a great piece of software.
I've never actively used it beyond the occasional `curl` commandline thing; have I subconsciously been using libcurl? Whenever I do a http request from idk, Java or Go, is libcurl involved?
Depends, it can be a http connection over tcp without curl being involved or it can be a curl request wrapped in underneath the hood, like in guzzle, for example.