Cant download previous versions of apps

Hi,

I just wanted to use my old 21.5 inch mid 2011 imac and i factory reset it. When i did that i upgraded it to highest possible mac os which is high sierra. But when i tried to install most apps from appstore like imovie i couldn't download it as it always gives an error which says i need to have at least mac os mojave. And as my machine is not capable of downloading that. I downloaded imovie 9.0 from apple's website but i wasn't able to update it through appstore to version that can be used in high siearra which. Can someone help me with that problem if there is a solution of downloading previous versions of apps. I don't want to dowload anything from unoffical websites even if the software is free but if there's a trusted one why not?