Having an .ipa file, is there any way I can get its source code?
No - the IPA format does not make source code inclusion mandatory, so there’s no general way to do what you ask. As alluring as the idea is, just because something is possible on Android (What you’re talking about is not in the sense you want it), doesn’t mean it will be possible on iOS.
Now, if the developer decides to embed the source code into the IPA - you will be able to read it depending on the method of packaging it since the IPA contents are well documented by Apple on the developer site and many utilities exist to decode the application bundles.
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