There is no single answer that will work for all target apps. You need to research each app and see if it has a facility for receiving info from other apps.
A simple way to do this is to invoke an URL that targets the other app.
For mail, you could invoke a mailto://
URL that composes an email with the text in the desired field(s) (to, cc, bcc, subject, or body.)
If the app supports the iOS document model you may be able to pass it a document to open.
If the target app has a server then you may also be able to connect to the server and send data to it that way. Again, this is not a question you can ask in general. The answer will be different for each target app, and for some apps the answer will be "you can't, because it doesn't have any mechanism to receive data from an outside app."
Android is a different beast with different abilities than iOS. iOS is more of a "walled garden", with very limited access outside of your app.
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