[How can i duplicate elementor page using divi builder.]

You can learn elementor easily from youtube or just goto dribble website choose any template and start clonning it, you'll learn it by yourself and if you got stuck anywhere just search it on google or youtube, its very easy.

[what is the best wordPress theme for websites?]

Depends on what type of website you want to create like, if you want to create a e-commerce website than use Astra, WP Ocean and many other, just search it on youtube, you'll easily find that what are you looking for or you can also find your self in theme segment. Some themes also offer prebuilt websites like you have to edit it only like your business info, your products etc.