The Sims 4: What are Disks 1 and 2 For? Can They Be Used Twice?

I got The Sims 4 as a gift (disks), and in the box came two disks, Disk 1 and Disk 2, and I've been wondering what they're for. I've heard that they're both so you can download the game completely as one disk holds some files, and the other disk holds the rest, or that Disk 2 has a bit of extra content like extra soundtrack. At first I thought that it included extension packs, but I saw that that wasn't true. But, I was able to download Sims 4 with only Disk 1, so Disk 2 has never been touched. My first few questions:

What is Disk 2 for?

Is it required?

What's in it?

Also, recently my friend mentioned that she has wanting The Sims 4 as she still has Sims 3, I told her about my two disks, so I told her I'd let her borrow the disks to see if she could get it on her computer. I don't know if she'll be able to download it as I already used one disk or if she'll even be able to use the code to download it since I already used it. So:

Can she still use Disk 1 and 2?

Can she still use the code even though it's been used?

And can she download The Sims 4 for her own Origin profile?

Please, only reply is you know the answer to this. Thank you!