Why is an alkyl group is denoted by #"R"#?
It originally was used as an abbreviation of the word "Radical"......but it is not only used for alkyl groups....
Historically, the word "radical" was used to refer to a part or portion of a complete molecule. It is less commonly used today, except in the case of "free radical" which is effectively a species formed from "part of a molecule" that has a free unpaired election.
However, the abbreviation R has stuck.
But R does not only denote an alkyl group. When you see something like
So R could be an alkyl group, but not necessarily.