It’s the StandBy screen. Your phone was probably charged and kept in landscape mode (which is how the StandBy mode gets triggered). At night (or dark environments) the widgets turn red. There are different styles you can choose from. You can also choose from a wide variety of widgets.
Anyway, if you don’t wish to see it anymore you can turn off StandBy in Settings.
I guess not. A friend had bought a new 12 mini 64GB for around 250 euros last year.