This is digital photography. Dark pictures get noisy, which can look like pixelation. A concert stage is usually a combination of very brightly-lit areas and areas that are almost totally black, so you're probably seeing sensor noise in the dark areas.
No, that has nothing to do with pixelation.
You probably never noticed it, or the metering is different, or you never shot dark areas at high ISO.
No, that also has nothing to do with pixelation.
That could be one cause. Keep the ISO as low as you can. You don't need much sensitivity for a spotlighted performer, and you're not going to get the dynamic range to shoot a scene with a spotlight and dark areas and get a good picture without using film and using tricks to extend the dynamic range of the film. Normal hi-contrast film as about 5-7 f-stops of range, but spotlighted singer to drummer in the shadows might even exceed the 14-15 f-stop range you can force film to. (F-stops are logarithmic, so 1 stop more is twice the light, 2 stops more is 4 times the light - 15 stops is moonless cloudy night to glaring sun.) Keep the ISO as low as you can for the spotlit areas and don't try shooting dark areas. Phone cameras aren't low-light cameras. (If you can't help getting dark areas in the shot, even at maximum zoom, you have 3 options: you can just accept the noise in the dark areas, you can buy a telephoto lens, so you can get 'up close" to what you want to shoot, or you can post-process the video - load it into a PC and use a video editing program to cut what's there to just the spotlit performer.)