I'm going to start with an entry-level mountain bike. I have seen Xidesheng Daily 700-A, Giant ATX730 and Duke Merida 600. Which is better?

I won't say much. I ride 700A a day, and I can't go wrong: the oil disc brake with 27-speed variable speed damping belt locks up and fine-tunes black and yellow. It's a good suggestion to buy by the day, and the price is relatively cheaper. You won't regret buying a card and smashing a car.