In general, multicast (natively - however, it might be contained in VPN tunnels running across the Internet) is not done across the Internet. If it's done at all (natively), most likely only within the confines of a single ISP. (BTW, keep in mind some ISPs are very large, so as long as you and your other multicast peer are using the same ISP, maybe. Again, unlikely, though, More likely, if you want to do actual multicast across the Internet you'll do it across some kind of VPN.)
Most "multicast" type applications, running across the Internet, are using unicast.
Sometime the "multicast" type applications, using unicast across the Internet, are being served by a 3rd party replication servers, i.e. a unicast stream is sent to the provider, they in turn send more unicast streams to their servers scattered about the Internet, the end users get a unicast stream from one of those servers.
Year ago, one (international) company I worked at did (regional - Americas) CEO to company "broadcasts" across the Enterprise. (Expectation/requirement ALL employees would view.) For internal network connected employees, we did use multicast (including, for a while, also using satellite). For our employees not on our internal network, we set a copy of the video stream to a (large) Internet replication provider, and our externally connected employees, got a unicast stream from one of the 3rd party replication servers.
As to applications that are "multicast" like, when not on the Internet, then they might actually use multicast.