When designing a TabView you have the option to add an icon for each tab using a system image, using the systemImage parameter to Label:

TabView {
    Text("First")
        .tabItem {
            Label("First", systemImage: "tray")
        }
}

In this case we use tray.

How do you find out which other icons you can use?

Apple created an app for this, called SF Symbols. Download it from https://developer.apple.com/design/downloads/SF-Symbols.dmg, install it on your Mac and once you open it, you can browse over 1500 symbols and see their name: