Thank you!

I noticed the 2.0 has 2 mod folders, I'm assuming the mod folder in root is where we put our custom plugins?

Correct

What's the difference between root mod folder and vendor mod folder?

The one under vendor/ is necessary only because Elgg comes with bundled plugins. If we would move each plugin from Elgg core into their own individual plugin projects, there wouldn't be need for the mod/ directory under vendor/. Composer would then simply install them separately into the root mod/ directory.

Initially there were plans to do exactly that, but we were afraid it would cause too much trouble: https://github.com/Elgg/Elgg/issues/5945 so we decided to make symlinks instead: https://github.com/Elgg/Elgg/blob/2.0/engine/classes/Elgg/Composer/PostInstall.php#L63

Sounds great to me, as I was old user of Elgg but left and started using paid softwares but I am back to Elgg as Elgg is the best in the whole market, its free , fast & support is very active.

I am going to use Elgg again for my new site, I am just waiting for the final release.

Author, please make a section where we can read all the features of the core software, it will help new users as well. Maybe this section is already here somewhere but I could not find.

When Elgg 2.0.0-beta.2 will be ready officially to use for a live site? I am getting so excited and looking forward to use it.