In what resolution should design my mobile game?

I am creating a mobile game for both IOS and Android, and I am running into trouble deciding what resolution to design it in. Would I run into issues if I created the game in the highest resolution out there (iPhone X) and scaled it down from there? And would creating the game in, for example, 16:9 aspect ratio bar me from putting my game on devices with other aspect ratios?

Did you try googling it?
Check out this Unity Answer, for example.