"Real" open days and tours can be biased too! They often only have selected accommodation blocks open for viewing and many of the lectures/presentations are in the more attractive/newer blocks, given by the more "personable" staff. We went to several open days, but still had to do lots of online research etc to see what we really wanted to see (such as virtual tours of the accommodation blocks that were closed), and also to get a better idea of the local area/town centre, etc.
We enjoyed the "real" open days, but there was a lot of wasted time (queueing, duplication of info in lectures/sessions) and some of the tours were quite long and tedious, sometimes with very little "real" information. A special mention has to be made of a Physics professor in Leeds who "ran" a tour where he basically hadn't the foggiest, he took us up and down corridors only to find locked rooms - after half an hour, we'd not actually seen anything except corridors, then he took us back to the meeting point to ask someone else where he should go, and then we trailed around the corridors again to finally find a lab with some students doing experiment presentations, but by the time we got there, the next group had already arrived and there wasn't enough room for us all, so we just gave up and drifted off!
But yes, you do get a "feel" for the place, probably moreso from the students (student ambassadors they're usually called), who can tell you the "reality" of life on campus. In some, especially Newcastle, they were remarkably friendly and honest, others, maybe not so much.
We also went to a few "offer holder open days" which we found a lot more useful as they're usually held on "normal" working days, with real staff and students milling around doing their "normal" daily activities, so you can see how busy it is (or not) when it's not as full as the special weekend open days.
I'd say doing both real and the online ones is best if you can, but if real open days aren't possible, either due to covid, or distance, the online ones are a pretty good alternative, but just try to get to offer holder open days if you've not been to the full open days.