So got a customer here with an AVD environment with no Nerdio or Hydra or anything - if they've got a scaling plan turned on, how do those machines get provisioned? bear in mind I'm a total AVD noob

With the existing scaling plan - you create and deploy session hosts, seperate to the scaling plan. The new method (in preview) creates session hosts for you https://learn.microsoft.com/en-us/azure/virtual-desktop/autoscale-create-assign-scaling-plan

So does it create them by cloning an existing snapshot or something, in the new method?

I haven’t looked in detail to be honest, but it will deploy from an image in the marketplace or a compute gallery

Dynamic Scaling Plans (preview). Does not support Ephemeral disks. The other one is really a power on/power off/hibernate scheduler.