As this repository provides a Package.swift on the root level, so you can add zxing-cpp including wrapper code by adding a Package Dependency in Xcode.
An alternative way is to check this repository out and add it as a local Swift Package by adding it as a dependency to your app.
You can also use CocoaPods. Just add the Pod as a dependency:
pod 'zxing-cpp'
The module to be imported is named ZXingCpp. If you just need the core without the wrapper code, you can use:
pod 'zxing-cpp/Core'
For general usage of the ObjectiveC/Swift wrapper, please have a look at the source code provided in the demo project.