In Settings —> Approvals invoices can automatically be routed to each Hiring Manager for approval. You must designate a Hiring Manager for each Vendor and you can also specify a different Hiring Manager for each Work Order.
If automated routing to Hiring Managers is turned on, then the Hiring Manager for each Invoice is automatically an approver for that Invoice: that will be the Work Order Hiring Manager if the Invoice is tied to a Work Order, or the Hiring Manager for managing the relationship with the Vendor if the invoice is sent without a Work Order.
Once invoice processing is turned on, one or several Invoice Processors are selected (at least one person must be designated as an Invoice Processor). Companies usually add their AP person, accountant, or bookkeeper as an Invoice Processor.
As always, only people set as Administrators, Controllers, and Invoice Payers (roles with permissions to pay Invoices) will be able to pay invoices.