i mean you can technically stay in the park hour(s) after closing if you're in line for a ride/attraction before the posted park closing time, but as soon as you finish you have to leave the park
Before the pandemic, I know that they'd usually keep the Main Street shops at MK open an hour after close, but now, I don't know if that's currently still the case (I don't think it is).
The parks will begin closing things down at closing time. Shops used to stay opened and transportation would run an hour past. Now with NBA having privileged not available to regular guests, probably at close.
We had post park dinner reservations at all parks except AK each night—- and a few times we were among the last folks out of the park— they were pretty serious about it— there were folks “nicely” waiting for us to guide us out of the park