Android and iOS are very different in this regard. Android is very open about letting you use intents to mix and match activities from other apps. iOS isn't like that; each app runs in its own private space and generally cannot even see other apps' data, let alone use their services. It is possible to cause other apps to run via custom URLs, but that's not the same thing.
What you'll need to do is to copy or move the code for the view controllers, views, etc. of your various iOS apps into your tab bar app project. You'll build them all into a single application, and the tab bar will switch between the view controllers.
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