both flu and covid kill the same kind of people, very old and frail ones. So in a sense they should be competitors, and they'll both kill fewer than if either disease were the only one.
Covid kills people who would survive the flu, so if someone catches both, they would have a higher chance of dying than from the flu alone, which would affect the flu's stats because these would be recorded as flu deaths.
Flu kills children, covid-19 doesn't. Not sure if there is an 'expected' percentage of flu deaths in children, but if so, we might estimate the covid effect by looking at those numbers.