Hi

Yes its all given in the document.

-->unless upgrade fails you no need to start updating IAAS separately I beilive .

Start updating VRA appliance that would update IAAS during Initializing then you could check the cluster back .

As you have external VRO, post VRA upgrade its good to upgrade VRO as per the document :

You must upgrade your vRealize Orchestrator instance when you upgrade from vRealize Automation 7.1, 7.2 to 7.3 or 7.1, 7.2, 7.3 to 7.3.1.

https://docs.vmware.com/en/vRealize-Automation/7.3/vrealize-automation-71or72to73-upgrading.pdf