Of course, most of the time pilots get to fly the expected STAR, and, of course, only a few of the many STARS and SIDS published are the ones used by most of the traffic. Most airports were built with the prevailing winds taken into consideration and most of the configurations are predictable (noise abatement procedures may influence this). If any aircraft have fuel issues (and many times without it, just to expedite the sequence) it will be provided vectors/shortcuts. And, at least in my country, an ILS Z approach clearance will be issued if available unless the ILS is out of service for whatever reason or unless the crew requests another one. I find annoying the many times I am issued a VOR app clearance in the sim when an ILS procedure is available.

In practice, most of the STARS are not flown in their entirety, since, once separation is achieved, the next traffic in the sequence will be cleared to the IAF regardless of its position in the STAR.

@IRLATC ILS Z/Y, which one to choose and why? I heard that Z has higher priority over Y. But not sure when to choose which.

I don't know the answer. I believe Z is the most standard one. In the letters of agreement between approach and towers, in my country, it is assumed that the ILS Z approach will be instructed. In case another approach (VOR, DME, RNAV, GBAS or a different ILS) is instructed, approach must coordinate it with TWR. Many times the difference lies with the missed approach procedure.