Hello Darshan
Frame-Relay is a layer 2 technology that can be used to interconnect multiple remote sites of an enterprise. You are correct, that the Frame-Relay cloud, in reality, consists of a large infrastructure of interconnected devices. When labbing this up, the Frame-Relay cloud is represented by a single device, so your understanding is correct.
As for your second question, the choice of technology to interconnect remote sites is based on several factors including availability, performance, and cost. Frame-Relay is a technology that was very popular in the 1990s as it was a cheaper alternative to leased lines.
Over the decades, it declined in use and has been largely replaced by other WAN technologies including MetroEthernet, MPLS, DSL, Cable, VPNs, wireless bridges, and fiber optic links, or some combination of several of these technologies. Once again, the choice of the WAN technologies used depends upon availability, performance, and cost, and today, most of these mentioned technologies outperform Frame-Relay and are also cheaper, and more widely available.
So to answer your question directly, yes, we could use Ethernet as a WAN technology, and in many cases, it is a good choice. However, there are quite a few other choices as well that may be more beneficial for your particular arrangement.
Why are we still talking about frame relay even though it is almost completely phased out today? Take a look at this post for the answer…
I hope this has been helpful!
Laz