How to modifies the APK and add toast or message

There is a site that modifies the APK for some programs and applications and adds a message like the one in the picture, how does it do this

Why not ask there?

I want known How can do it
Do you know

Don’t upload your app to such site’s they modify your app and remove ads and put their custom tracker of user details

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I do not do that . All I need to know is how they can do that

Like i said ask them. I will close this topic.