Visit Orvieto on Sunday?

Orvieto's tourism website provides this sort of information:

https://www.orvietoviva.com/en/

I'd definitely trust the tourist office's website or (even better) the websites of the sights themselves over information from TripAdvisor or Fodor.

I can't say for sure about the ceramic shops, but it's my impression Orvieto gets a lot of Italian tourists, and I bet many are day-trippers from places like Rome. Weekends are busy in Orvieto. Maybe use Google Maps to identify a few ceramic shops and then look for their websites. However, you might run into the same issue I've had with small restaurants: little info on the website (or no website), with info about changes in hours of operation mainly showing up on Facebook.

...or (even better) the websites of the sights themselves

Wise words from acraven, IMHO, and true for pretty much any attraction. Unexpected events can change closure days (or entry fees and hours) on a dime so the most CURRENT info is apt to be on their own websites and not TripAdvisor or very old forum posts. I just used the Orvieto tourism site as a shortcut to get you started.

I agree with the others about checking individual websites. Here are some links for you to start off with.

Duomo https://www.opsm.it/duomo-di-orvieto-visite-e-orari/

Orvieto Underground http://www.orvietounderground.it/

Archeological Museum https://www.musei.umbria.beniculturali.it/musei/museo-archeologico-nazionale-di-orvieto/

St. Patrick's Well No website but there is current info on times from Google Maps. https://maps.app.goo.gl/uuDmVzXExAergTf59

Ceramic shops This site will show you a number of shops, then you can click on each link to find more detailed information about shop hours. https://www.orvietoviva.com/en/category/orvieto-ceramic-shops/

Thank you very much! Your links are very useful.

May I ask how long to tour each site? What is the reasonable order of visit for those sites on Sunday?

Duomo: Sunday 1:00-4:30pm
Underground: guided tour every day at 11.00; 12.15, 16.00; 17.15
Archaeological Museum: Tuesday to Sunday 8.30am – 7.30pm (last entry 7.00pm)
Well of St. Patrick: 9:00-6:30pm
Ceramics shops: 9/10am-7/8pm

here is my plan
Archaeological Museum at 8:30-10:00am for 1.5 hour
Ceramics shops: 10am-12:00
Underground: guided tour 12:15-1:15 for 1 hour
Duomo: 1:30-3:30 for 2 hours
Anything else from 3:30pm?

Well of St. Patrick (ignored, too many steps for me)

Duomo shouldn’t take 2 hours

But the duomo museum and Etruscan museums are also worthwhile. Those who like modern scultpture should check out the Emilio Greco Museum.

I found that the duomo museum and Etruscan museums both are close to the Duomo. But ceramics shops are somewhat far away. In addition, I am not interested in buying ceramics. So how about this one

Archaeological Museum at 8:30-10:00am for 1.5 hour
the duomo museum: 10am-11:00 (too short?)
the Etruscan museum: 11:00-12:00 (too short?)
Underground: guided tour 12:15-1:15 for 1 hour
Duomo: 1:30-3:00 for 1.5 hours

I found this description about Orvieto

Orvieto has a peculiar position, perched on top of a tufa hill in Umbria, central Italy.
The hill is visible from the motorway and, when you first see it, chances are you will second guess what you are seeing: is it possible it is houses you see up there and even a church? Indeed, it is!

Is the first picture in the link below the view of the town? Do you know where I can get such view?

https://www.orvietoviva.com/en/orvieto-porano/

yjyung, no one can tell you how much time you will spend at any of the attractions you list. You just have to pick what interests you and go with the flow. Just make sure they are open on the day you will be in town.

And find your own interesting viewpoints for photos.