Likewise, what are you upto these days means?
It means "How are you," and you respond with what you have been doing in the past few days, what you are doing presently, or what you will be doing in the near future (next few days). A.S.C.M. Monday, October 13, 2003, 21:51 GMT.
Secondly, what is meaning of what are you doing? Used to ask what the interlocutor is currently doing.
Herein, what are you upto these days reply?
"What are you up to" is a very common and casual (informal) way of saying "What are you doing?" You may also hear people say "What's up?" which has the same implication. Your answer to this should be equally casual, as most people don't expect a long or detailed explanation of what you are doing.
Is it this days or these days?
What is the correct grammar, “this days” or “these days”? These days, because days are plural. Unless you are referring to a particular day, then you could say 'This day'.