AVAssetExportSession

What do you want to know exactly ?

Here is Apple's example on how to use it:

h ttps://developer.apple.com/library/content/samplecode/avexporter/Listings/Swift_AVFoundationExporter_main_swift.html

And a detailed tutorial, to explain all steps:

h ttps://www.raywenderlich.com/188034/how-to-play-record-and-merge-videos-in-ios-and-swift