See section 3.34 in the manual (p. 70).
https://www.reaper.fm/userguide/ReaperUserGuide570d.pdf
It talks about recording an overdub over a looped section, but the same principle applies to just playing the looped section on repeat.
EDIT: On second look, I see some other sections that might be helpful also: 7.6, 9.3, 9.13.3, 13.13