Unit number on the interface is just a label that is only locally significant on that interface. The only configuration requirement is that you cannot repeat the unit number on the same interface.
There is no requirement that the unit number and vlan id match but on trunk ports many companies do that as a matter of convention.
I've seen automated provisioning system assign sequential unit numbers and then configure whatever vlan id is needed on that unit.
And since the unit is only significant on the specific interface, there is no requirement for the same unit number to be used for the vlan on different ports or different switches.
As you noticed, there are multiple ways to create a trunk port. You can use whichever method or methods you like.