The answer is slightly different whether you are using Cloud or OnPrem 

I'm assuming you are usingOnPrem

There are two ways to dynamically route task, either using Teams with Roles or just using variables.

To assign with the variable, a field containing the data must be in the form, so in the case of assigning to an employees manager, you would need two fields, one capturing the Employee name (Current User) and a second field populated by a table of some sort that would contain the Manager name that would be populated based on a lookup of the Employee Name. The challenge hear is you must update the Table as employees change.

Should you choose to use Team/Roles, then you can update the Role assignment in Forms Administration > Teams Management. You can assign the Person using Teams Assignment and then creating a Filter to assign the Role. In the Case you would assign the Employee to a Role such as Depart A, and Depart A Manager.

Either way it takes a little planning to make this work.

Hope this helps