Typically I use the activity monitor to analyze the file content. There is always 1 lib at least that a specific framework brings along, even in case of native programs.
When that is not enough, you can also get some cues from the Accessibility Inspector to "guess" what a program uses.
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[ 4.4 ms ] story [ 17.4 ms ] threadWhatsApp seems to use UIView, but I couldn't find evidence of SwiftUI, so am assuming its just a UIKit app.
There are some very similar UI components to the mobile app, so I am guessing they want to unify as much as they can between the two codebases.
WhatsApp Windows seems to be a native Windows app.
When that is not enough, you can also get some cues from the Accessibility Inspector to "guess" what a program uses.