Design the Sidebar

Organized

Structure the sidebar properly with correct titles, make sure every item is categorized to avoid confusion. For example, Mail omits the word Messages from the title of each mailbox, using more concise terms like Flagged and Drafts. Also, make use of the list view when the data hierarchy is deeper than two-levels.

The less you see, the more you can focus on a few things at once. list view gives breathing room. Don't overwhelm your screen with too much structure or unneeded visual elements.

Customization

Most importantly provide users with the ability to customize the contents of a sidebar. Why? because what may feel important to some may not to others so provide shortcuts to the content they care about so that it is more accessible to them. The more freedom you can give the users to choose between the better.

Clarity

Make things obvious. Buttons should be self-explanatory and typography should be big and readable at a comfortable distance. Your content should clearly indicate what your app is about. For example, if it's a email app, then you should be showing all the inboxes, groups and accouts to switch.

Having a Sidebar

Having a sidebar makes navigating your app a breeze as it contains all the necessary shortcuts and links. If your app has multiple levels of navigation and contents it is suggested to have a sidebar. If your app has not more than 5 section of content make sure of the Tab bar.

Displaying the Sidebar

Allow users to hide and show the sidebar whenever needed. By hiding the sidebar users get more screen space and when needed they can simply show the sidebar using the built-in edge swipe gesture or using the icon at the top-left corner. Make sure that the sidebar is not hidden by default.

Gesture

Adding a gesture can be an extension to an already visible interaction. It makes it easier for power users. We're all becoming power users, that's why we require less visual cues and more useful functions. For example, you can use the swipe gesture to allows users to quickly show and hide the sidebar menu from the app.

Apple's Human Interface Guide on designing Sidebar

There are many exciting guides and videos in Apple's Human Interface Guidelines that explain the Sidebar's development and design process in detail. Also guide on using the "UICollectionLayoutListConfiguration.Appearance." in SwiftUI.