In the past, I've walked up to a restaurant on Citywalk, asked for a reservation, mentioned where I was staying, and the wait time (even with a large party during busy times of the year at popular restaurants) has only been about 20 minutes. I usually take that time to browse their gift shop. Of course, I also usually eat at earlier times. We typically eat around 5pm.
In the park, once the restaurant is open, you can make a reservation that day. Again, I have gone there with a 7 person party at a popular restaurant during busy times of the year, and gotten in (Mythos, Cowfish, Anjolitos.) I was told that Universal doesn't care for people who are not already in the parks making park restaurant reservations because of people that don't show up.
Having done both Disney and Universal, I think Universal is kinda hard for the Disney folks to wrap their heads around. In order to have fun at Disney, you typically have to have every hour of every day planned out in advance - park, meals, ride reservations, etc. The parks at Universal are much smaller. Walking from one end of the other, not to mention one park to the other, is not much of a chore (even if you're out of shape like me.) Walking to Citywalk is not a chore. So setting an alarm on your phone to head to your desired restaurant when it opens (typically 11am) to make a reservation for that day is not a chore. In fact, during normal circumstances, more often than not you could easily walk up to your restaurant of choice when you're ready to eat and get a reservation with less time spent waiting than you'd spend waiting for your food to be prepared at a quick service meal location.
That being said, I have never found priority seating to be a thing. I have actually had restaurant staff (Citywalk restaurants) look at me in confusion when I've mentioned it. For the restaurants that take reservations, you can easily make them at the guest services desk in your onsite hotel, so perhaps that's why it's called priority seating? I think there is also a reservations desk set up outside of Confisco Grill, but again, the reservation policy really depends on the restaurant. It is always easiest just to head to your restaurant of choice on that day and make a reservation, especially for in park restaurants. Think of it kinda like the VQ process for Hagrids, as most of the slots are done by reservation by people actually in the park or on Universal property that day, with some reservations each day left available for walk-ups, and others becoming available when people don't show up for their reservations times.
I haven't been to the parks since they reopened, so I'm not sure how COVID-19 will affect this. But with limiting overall park capacity, I'm not expecting it to suddenly become an issue once they have time to work out the kinks and get used to the "new normal." We easily got a table for 4 at Finnegans at 4pm in September during
HHN. Finnegans is considered prime seating during this time because of it's ability to sit down in the AC, have a drink, and just relax while being positioned next the popular Houses while waiting for Universal to clear everyone (non HHN attendees) out and HHN to start. So, I know things are a little crazy right now, but I'm expecting it to be fine.
I will be there tomorrow, so we'll see.
