Apps with the key symbol in the corner are signed in through Classlink. Classlink is your username and password.
Apps without the key have a separate login, usually Google. Once you log in for the first time, Classlink will still remember and save your login info.
The first few apps in your Classlink dashboard include Carroll's AUP, a link to your Aspen account, the "first six weeks" guide, apps you are already rostered to, and this tech help website!