Can you use Divi Builder with the Astra Theme?
Yes, you can use Elegant Theme's Divi Builder functionality on your Astra Theme pages and posts. To do so, first install and enable the Divi Builder plugin, along with the Astra Theme. Then when you start a new post / page you should see the option to "Use Divi Builder", like so:
Does Divi Builder work with the Astra LearnDash Starter Site?
One way to set up a LearnDash site on Astra is to use the preconfigured LearnDash "Starter Site". You don't have to use this to use LearnDash with Astra, but it's an easy way to get started.
However, note that this site is built with the Elementor page builder and as such isn't compatible with Divi or the Divi modules for LearnDash added by my Divi LearnDash Kit. You will still be able to create new pages using the Divi Builder, but you won't be able to edit the existing Elementor ones in Divi Builder.