Mobile first approach

Hi, as i finish the ultimate html and css course of Mosh Hamedani. i want to practice what i learn so i first create this easy challenge then will go next level next. I did this by mobile first approach but as you can see it has a problem in the breakpoint layout which the layout becomes ugly and i don't know how to fix it i also have problem applying a border radius to the container. I also practiced and tried how to write a maintanable and reusable css which use object-oriented and bem . any suggestions will help me a lot to improve.. Thank you