How to study app development from the scratch?
Welcome to the forum!
Are you referring to mobile app development?
If so, then you can try these sources.
If you are referring to web apps then you can start with the FCC curriculum.
Hope that helps!
If you are a beginner and want to learn from scratch checkout this blog-
And if you want to learn Android app development, you can read this one-
I hope this all will help you.