This is a generic answer for anyone thinking about careers developing iOS, MacOS, and similar applications.
Hacking with Swift is the number one resource for diving into the application development pool. @twoStraws paces his lessons from absolute beginner through to working applications. And it's brilliant how he'll show working code, code that you or I would write, then a few lessons later teach a slicker, more concise, swiftier way to write the same code.
You build your knowledge upon previous lessons.
Development Teams
From the gist of your message, you're a hobbiest? You're crafting you own applcation to share with family, co-workers, or perhaps push to the Apple App Store to bolster a resume? Nice!
In many larger organizations, developers and interface designers work together. But their jobs are separate. A developer will have the freedom to design logic, data flow, message types, json structures, business rules. But they will be given explicit instructions how the interface should look.
You're making an application for a Uni? Here are their school colors and logo. This is a local green grocer's application? Here's a high resolution image of their market, and here are some seasonal vegetables. Use these! You'll even be told how much blur to put on a logo, what opacity to add to a background image, and just how rounded to make the text field's corners. You may not even have a say in the order of the fields on a screen. That's a designer's job, not a programmer's.
Of course your team may work differently. But this might explain (a bit?) why @twoStraws' lessons are focused on application functionality, and a little bit less on look and feel.
User Interface
Now, think of the same type of lessons and objectives, but only for the look and feel of your application. This is almost surely an entirely different career path! There are years of lessons with complementary and contrasting colors. Dozens of homework assignments on shading and opacity. You'll take back-to-back terms learning fonts, learning about legibility and setting moods.
@twoStraws really doesn't touch but the surface of some of these topics.
Interface Design
You can start with the Google. Perhaps the key word you want to search is UX. The term UI brings you to many topics about how to make buttons, swipe gestures, and text boxes. The keyword UX describes the artistic look and feel of applications.
See --> UX for iOS 15
Also, take a peek at Pinterest, of all places. You'll find some clever collections of ideas there.
