I understand that "dou" means how, and for example おちゃはどうですか means that I'm asking how is the tea? But, in Busuu app it says that I can use the same question to offer something, as if I'm asking "how about a tea?" I'm not really sure about this last thing so I hope you guys can help me.
Yes, ～はどうですか can be used in several ways.
"How is ～?"
How is he doing recently?
How is Japan? / How do you like Japan? / How is your stay in Japan?
"How/What about ～?" (suggestion, offering)
How about some tea?
How about (going to) Japan? / What about Japan?
"How/What about ～?" (asking the same thing for another topic)
I'm fine. How about you?
It's very cold here. How about Japan?