How to gain access into /etc/networks/loopback file from terminal?
/etc/networks is not a folder, it's a plain text file with the contents given in your question.

loopback, 127 and loopback-net are not files within a file, they're just words within the contents of the /etc/networks file, as part of the configuration expressed within. There's no /etc/networks/loopback, as it's just the file /etc/networks with the word loopback within.