Turnitin.com is a separate system from other University systems (Office 365, ICON, etc), and requires you to set up a separate account. Please refer to the link below for instructions.
No, you cannot submit a Turnitin assignment using the Canvas app from a smartphone. You need a tablet, laptop or desktop computer to do so.
No, students cannot submit multiple file uploads. They may be able to resubmit a document depending on the assignment settings, but cannot upload more than one file at a time.