Do kitchen undermount sinks always attach to the granite / quartz or do I have to make a precise hole in the 5/8" plywood
I have installed sinks that are supported by the plywood sub base many times. Sometimes it was framing added strategically, since there was no room for a plywood base.

FWIW, countertop installers, all of them in my experience use bolts epoxied in slots under the stone top. This is for 3 cm stone, I would not want that in 2 cm stone, then I would only use plywood or framing for supports. That is my preference.

I have had some installers complain for my base material being in the way for their bolts, my miscommunication. Be certain how the sink is to be held in place so there are no problems during install.