Input validation and calculations using Javascript

What are you most proud of, and what would you do differently next time?

I'm proud of how I approached the challenge by splitting it into smaller parts. I'm also proud that I did it myself.

What challenges did you encounter, and how did you overcome them?

I thought this challenge would be straightforward, then I realized there were many details to consider. At first, I found it overwhelming, but then I wrote down my ideas as comments in the code, then I tackled them one by one.

What specific areas of your project would you like help with?

How should I fix the layout such that it still looks good even if the user enters large numbers (e.g. 1000000000000). Right now, since the layout is responsive, it breaks when the input is large.