Hi SyThai,
Thanks for the additional info and attaching those pictures! That helps to provide more context on what you're referring to.
So it looks like you're just comparing the appearance of the Mail application on a Mac, to the Mail application on an iPhone. While Mac and iPhone software does look quite similar in many ways, they still run completely different operating systems so not all options and layouts will be universal.
In your case, the layout you're referring to in the Mail app on your iPhone is just the way the iOS software is currently designed. The Mail app in iOS does not have the collapsing sub-folders like the Mail app on a Mac. It's not an actual problem, just differences in the design of the two different operating systems.
We can definitely understand how the less cluttered appearance in the Mac Mail app offers a better workflow when you have large amounts of sub-folders. Apple does offer a feedback page where you can submit suggestions for future software and hardware they may release. You may want to provide suggestions there for changes to the iOS Mail app layout. Here's their feedback link: Product Feedback
We hope this helps. Take care!