Design requirements of a big website and an application

Question

I want to design a big website to sell goods, what skills do I need to learn for this? (I know C# language and I am working on the database)
Also, what programs or books do you recommend for learning Android? (I want to create an application on cellphone as the website that I mentioned above .)

What steps should I take in order by mentioning the priorities:
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Answer 1

here some links you can check by your self
https://kotlinlang.org/docs/roadmap.html
https://flutter.dev
https://developer.android.com/guide