Banners are the hardest images to get right, because they are displayed at completely different shapes depending on the device.
A wide banner on a desktop becomes a much narrower crop on a phone, and the platform takes that crop from the middle. Anything you place near the left or right edge is invisible to over half your visitors.
So put the essential content, a logo or a line of text, in the central third. The edges are decoration and should be treated as expendable.
Upload at the size asked for. Platforms that scale your banner down do so with no regard for sharpness, and a banner you resized deliberately will look better than one the site squeezed for you.
Because mobile shows a narrower slice of the same image, taken from the centre.
Whatever the platform specifies. They differ, and they change the numbers periodically.
No.