Is It Common For People To Write Literature Reviews On A Topic For Someone Else?
I don't know how common/typical it is, but it seems to be certainly part of a job description for a RA, especially if your implication is that you are a doctoral student.
One reason for it is just part of your training and when it comes time to do your own dissertation the practice would help.
But additionally, if the professor is exploring some new research direction it would be helpful to them to have help in assuring that they don't miss something important.
And anyone starting out in a new research area will have the same dilemma as yourself. It is hard to know where to look initially. You need to review a lot of papers and chase a lot of references. That is the nature of the task.
If it were crystal clear it probably wouldn't be necessary.
It is good that  you are humble at the start. I'd guess that the professor knows that it isn't easy and that the initial product might require refinement. Just be honest along the way about what you have &quot;covered&quot; and any gaps you think might still exist.