What's the processing order for a gpo?

Group Policy always applied in this sequence LSDOU

It’s group policy inheritance model, where the policies are applied to Local machines, Sites, Domains and Organizational Units.

First the local group Policy is applied => then Active directory Site Policy => Domain Policy => Organization Unit Policy

Local policy is applied using gpedit.msc

and other policy like AD site policy, Domain Policy, OU Policy can be applied by using GPMC Tool called Group Polict Management tool .

or from Active Directory Site & Services and Active Directory User and Computer

Local GP then Site GPOs then Domain GPOs then OU GPOs

if there is a conflict the last wins,  GPOs are commulative