EverythingGirl.com was basically just a web portal for Mattel's various sites, such as Barbie.com, PollyPocket.com, BarbieGirls.com, MyScene.com, and more. At the top of everyone of these sites was a little bar which you could scroll through to access other sites under the EverythingGirl.com portal.
You could also make an account on EverythingGirl.com, and play some various mini games. I didn't use this feature, so I don't know much about it. There's not much documentation on it online. I think there was a virtual currency, and maybe you could raise a virtual pet? You could also earn points by playing mini games on the other sites that you could use with your EverythingGirl account.
Everything ran on Flash, with plenty of animations and even voice acting. The sound effects used are forever engrained in my brain. When you visited the site, the site mascot, this cute fairy called Pippa, would introduce herself to you. It seems a bit cheesy nowadays, but it was awesome back in the day!
EverythingGirl.com was phased out some time in the early 2010s, but I think the site still remained online for a while afterwards. You used to be able to see it in its full glory on the Wayback Machine, but now Flash is gone so nothing works right.
I think the site's design is super cool. There was one point in my life where the only sites my parents would let me visit were Mattel sites. For a long time I didn't have any games consoles, so most of my "gaming" was spent on places connected to EverythingGirl.com. There's aren't many decent screenshots, but I will put some videos below so you can see what it was like. (I did not make these videos.)