what is the difference between an RNAV Z/RNAV Y approach versus an actual RNAV approach?
In the case of Ottawa (and the rest of Canada from what I've seen) where there are two RNAV approaches to the same runway, the RNAV "Y" approach refers to the "RNP" (Required Navigation Performance) approach, while the RNAV "Z" refers to the RNAV "GNSS" (Global Navigation Satellite System) approach.
They are both RNAV approaches that use GPS/GNSS but basically the RNP (RNAV Y) is more precise. For example runway 25 in Ottawa, the minimums for the RNAV Y are 250 feet (agl) and 1 mile vs the RNAV Z minimums of 390 feet (agl) and 1.25 miles.
The lateral design of the approaches is also different, the RNP approach can utilize a shorter transition off the STAR (called a "shortgate"), and each have different Final Approach Fixes.