So, you have already watched every TV series ever released during the quarantine and now you dusted off your old PlayStation (or Xbox) and attempted to play that shiny new game you've seen on YouTube. But something seems to be off - the graphics doesn't look as good as on YouTube.
Things that could go wrong:
- TV that's worse than your other displays.
To exclude that possibility try to see the gameplay on that TV. If it looks just as bad - then it's bad TV (which may happen if you are comparing image quality on OLED screen of modern flagship phones against 10-year old budget TV). - Comparing graphics between different platforms.
Some games were released for PS3, then remastered for PS4, then got enhanced graphics on PS4 Pro. However, it's perfectly possible to play PS3 variants of some games on PS4 using PlayStation Now subscription. So in the worst-case scenario, you could be comparing for example footage from The Last of Us Remastered on PS4 Pro, against The Last of Us original release from PS3 (played on PS4).
To exclude such possibility make sure to compare footage of exactly the same release on exactly the same platform. - There could be signal degradation.
The cable or port on TV or port on the console could be dirty or damaged, resulting in not displaying the image correctly. You could also run into signal bottlenecking, when you are trying to use old (and/or Chinese) HDMI cables. Older HDMI standards (up to 1.4) do not support 4K, therefore if your TV supports 4K, the PlayStation content will look noticeably worse than everything else. To attempt to fix that disconnect and reconnect all cables, make sure the ports look alright and try a different cable if possible. - Your PlayStation could be overheating and/or physically damaged.
Modern electronics are "smart" enough to degrade performance when at the risk of overheating. Therefore let's say the PlayStation is indeed overheating and performance is degraded to 70%, but the system does not shut down. Many games are designed to display constant 30 frames per second no matter what demanding tasks are going on on the screen. Therefore they compensate with lower resolution. Usually, you don't notice that and once the action on the screen calms down the resolution is back to normal. But if the available hardware capabilities are constantly limited and the action does not slow down, the lower resolution can stay and you can notice it.
To eliminate that possibility make sure your PlayStation is not overheating - maybe it's standing right next to the wall with fan holes covered, or laying on the carpet, or the fans are not spinning. Just make sure it is cooled adequately.
But what the console is broken altogether?