The Dashboard is generally the first page you see when you login in Marketing Cloud. Although, this can be changed via Default Login Preference page.
Marketing Cloud Security Settings allow you to control login, access and roles and permissions for the users. Alos, allows you to limit scope for the APIs.
Sending Mobile messages through a Journey is not as straight as sending emails. For SMSs, we need Mobile Number in correct format & Locale for each contact
Mobile Studio enables marketers to engage/connect with mobile users via SMSs, push messages on apps and via Line to broadcast to a larger audience.