In Alphas, the game is still being developed: the models, the math, cinematics. Those usually start with a version number of "zero point (0.xx)" and this continues (0.1, 0.15, 0.3, etc) until the game is pretty much done, when it becomes version 1.xx. Some Betas may open before that, but that is where some developers have the near-finished game tested for bugs.

Closed Betas require you to be part of that company or a firm or invited to test the game and likely an NDA (Non-Disclosure Agreement- ya can't talk about the game to others). An Open Beta isn't so restrictive. Anyone with (whatever requirement, in my case I guess it was having an xbox) can join the testing. I logged a number of bugs for them to fix. Game review isn't part of it. Stress test having a bunch of people on the servers, cross platform interaction, find things the closed beta might not have found because there's just so many of us and everybody plays a little different.

The goal of the Open Beta is to fix all the stuff prior to the release date. Lately, that doesn't happen so much- examples being players clipping through the ground on Pokemon Scarlet/Violet, or servers crashing with... dang what was it, Overwatch 2?

Anyway, I'm sure I'm wrong on some of the nitty gritty, but that's basically what an Open Beta is and what was going on with D4.