What I'm saying is there is more to video than resolution. Viewing distance, codec, bit rate, frame rate, the screen size and technology all play a part in the perceived quality. Viewing distance in particular has a huge effect.
For instance, to tell the difference in resolution between an iPhone XS and XR you have to get the thing so close to your face making it an unrealistic use case, at arms length you really can't tell. But the XS screen is obviously far superior because its OLED so black is black. The iPad Pro has the same PPI screen as the older iPads but the Pro's screen is so obviously superior because its 120Hz vs 60Hz.
For video you also have to factor in compression. In the specific case you said, it will depend on how close to your face you are holding the phone, a 2" inches sure you'll tell but that isn't really normal. If the phone is propped up on a table 1M from your face, no chance. Most people can't tell the difference between HD and 4k blue ray on a 50" TV at 10 feet. But if you tell them one is 4k and one is HD they will always say the 4k is better because that's how people's brains are programmed by marketing.
On the other hand I stream YouTube at 1080P on my PC even thought the browser window is only about the size of 720P. I do it because the compression artefacts are obvious on 720P compared to down sampling 1080P. That's not because of the increased resolution, its because of the far higher bit rate (or less compressed video). But lets say I moved the monitor further away, I wouldn't be able to tell.
When you start comparing things like phones to iPad's there are other things to take into account like the general experience of propping an iPad up vs holding a phone a few inches from your face and LCD vs OLED. Likewise 1080P on iPad is using a different video source to 1440P on One plus, its still YouTube, but its a different video. Again it would depend on the distance because of the dramatic difference in screen size.
For pretty much everyone 1080P YouTube on an iPad at normal viewing distances is more than acceptable quality.