Mobile-first Flexbox approach HTML CSS solution

My approach

This is my first challenge that I have attempted using frontend mentor. I recently learnt about the mobile first development approach and wanted to try it out. I figured since the layout was simple enough, I could use flex for everything. I never used the dynamic values provided and have hardcoded them up. I could not get the design as it is because I did not have the Figma file with all the spacing, fonts so I have guessed most of things.

Some questions I had for the community

1. Is this approach feasible? While I jumped from mobile to a desktop version, I had to make quite a few changes as I had set margins and paddings for everything but I am not sure how scalable it would be if more features are added.
2. The mobile view is somewhat broken. I had given different heights to the section maybe that is the reason behind it.

If you could review and give feedback on the code, it would be highly appreciated :)