Flexbox have been used with position relative

Hello all , I'm proud of what I've done even tho I wasn't able to do use correctly the background bottom on desktop and mobile device.

Unlike my previous challenge I used more semantics tags so I think I'm starting healing my div disease x)

Could you give me some feedback and if possible a ressources where I could learn how to use background position like "Flexblox froggy or Grid Garden".

Thanks for reading !