dndbeyond.com handbook question

Hi all, I've never played DnD before, but am looking to be DM for a small group of friends who also haven't played before.

The searchability and crosslinking dndbeyond.com provides is really appealing to me as a beginner, but I'm a little unclear about what is and isn't included when purchasing these digital handbooks and adventures.

It lists the cost of the Player's Handbook as $19.99, but what I'm unclear on is what the "Additional Purchase Options" is below. Is this additional content not included in the handbook or is this just a way to allow users to purchase parts of the book a la carte?

Lastly, as a new player (and the group's DM), I imagine I'll need all 3 handbooks to get started. I want to start with the Lost Mines of Phandelver adventure. Would I still need the Monster Manual in this case?

Thanks for your help!