The Samsung Note 20 Ultra is rated at 1500 nits which is close to being one of the brightest phones available if not brightest. But if it's set up in past Samsungs like my Note 8, you'll never see it pushed to 1500 in probably any indoor scenarios. Indoors the phones max bright will probably be in the 700-800 range done on purpose. Without Adaptive Display on, Samsung will probably set a fixed max somewhere in that range.
I linked a video so show but you have to set Adaptive Display to auto and only then and you're in an outdoor situation where it is crazy bright where with past phones I had, impossible to see the screen in such bright sunshine or squinting to see the screen, the Samsung will push the phone to it's 1200 nit max brightness and living in the sunny South Florida it comes in handy. The first phone I've owned that I can see everything vividly in such a scenario and it is noticeable that it does get brighter and pushing to that 1200 nit level than when showing at max setting indoors. But I don't have one of those sensors to give you real numbers to back it up.
I am only guessing that Samsung is still doing this and what the OP is seeing when comparing phones indoors is the max brightness indoors.
Samsung should take note in future flagships... "Flagships absolutely have to be 10% brighter than our lower levels otherwise people will think the lower spec screen is as good or better... even though the flagship has higher resolution, higher refresh rate, etc."
I do get bothered if a screen is not bright enough so I hope Samsung didn't set it's brightness level too low. I haven't actually seen the phone in hand like the OP so I will have to wait until mine gets delivered to see if its acceptable or not. But right now I haven't seen or read complaints really so I think it should be ok.