The source code is compiled when archived for upload to the store - the resulting .ipa (zip) file will contain a 'strong' app store-encrypted binary, along w/relatively easy pickn's icons, plists and any databases/data sources included in the bundle.
If you'd like to see for yourself, locate an app/.ipa (do a Finder search on: .ipa - usually they're in users<youname>/music/itunes folder, as an example - select, duplicate and drag that fresh .ipa somewhere where you can work with it). Change the file suffix from .ipa to .zip and uncompress - this will result in a folder you can browse at your leisure.
Reference: https://www.apple.com/business/docs/iOS_Security_Guide.pdf