Can they still use it?

So I logged into my icould account from a friend's Iphone to do something but I made sure to sign out at the end. However, I could still see their phone's name on setting but that doesn't mean they can use it or see what i'm dowloading right? Sorry but i'm still new with the whole Iphone thing! Lol

If you stay logged on to their iCloud or iTunes accounts, everything you do they will know about it as far as downloaded apps. After you log off their accounts and back your own accounts, they will not know what you downloading.

Shouldn't be able to, but if you want extra assurance delete that device from the associated Apple devices.

When in doubt, change your Apple ID password. That disables every use of your Apple ID, so you'll have to sign in again where you still want to use it.

Next time, don't make changes to your buddy's phone, just visit iCloud.com through Safari. Once you're logged out there, he can't do anything else with your account.