Which auto assignment? :)
New tickets have a default department set in:
Admin panel -> Settings -> Default Department
Re-opening tickets has an auto assign to last respondent has a toggle set in:
Admin panel -> Settings -> Tickets -> Re-open Tickets
Help topics can auto assign tickets to staff or teams in:
Admin panel -> Manage -> Help Topics -> (click on each one and check auto-assign to)
Ticket filters can also auto assign based on selected criteria in
Admin panel -> Manage -> Ticket Filters