Does -io such words like 'scenario', 'oratorio' is considered to be a suffix or a part of a root?
By certain standards, it can be considered to be a suffix (in English). It shows up in a number of words, at greater than chance frequency. If it is a suffix, you expect to be able to find the putative root in other words that don't have that suffix: radio ~ radiation ~ radar; audio ~ auditory; scene ~ scenario; physio ~ physical; mustache ~ mustachio. It has no identifiable semantic contribution, but a lot of (supposed) morphemes of English don't actually have identifiable meaning ("cran-", "per-"), it is not productive (but neither are -ren or -th). The answer to the question whether it can be so considered is "you definitely can". There is not a consensus on a theory that tells us whether we should consider it to be so.