This is a pretty good explanation of what is going on.
What you can do is give your TV provider a simple phone call and start yelling at them why those greedy b🤬astards don't supply a high quality video stream with more bandwidth as you're paying far too much money for their service and you're not willing to watch pixel games on your screen for that price.
Jokes aside: Today's videos come in a compressed data format that comes with the costs of loosing some quality in audio and video. If you wouldn't do that a movie won't fit on a DVD disc and your interest line would be too slow to transport that amount of data fast enough.
These compressioethods work in a way that they leave out those aspects of the material that you will hardly notice. So instead of remembering the exact colour of every single pixel in that (black) square they simplify it to "there is a square patch of that size and colour at this position in the picture", because it's very unlikely that you will notice with the human eye. Same is done with audio. Stuff that you can hardly hear will not be transmitted.
That's exactly what you can see in your picture.
To reduce this you cannot do much. First thing is to use more data per second (bandwidth) for storing/transmitting your data. This costs money and maybe even limited by your internet connection so you would maybe have to improve this as well. A second approach is to do some post-processing on the picture: those square patches will be blurred with the ones surrounding them and become less visible. (To say it a bit simplified). Good media renderers are usually capable of doing this. But this will still not improve the quality, just hide the bad quality a little bit better.
It may also be as simple as your HDMI wire 🤷♂️
1)) Use better and higher quality video sources.
Buy a better TV that has better Picture Processing. ie modern Premium Sony, Panasonic or Samsung.
Don't sit so close to the TV !