Migration configuration
What I did:

I defined a field entry in the source:

  name: hardwares
  label: 'Hardwares'
  selector: hardwares


In my process:

_products:
    plugin: sub_process
    source: hardwares
    process:
      products:
        plugin: sub_process
        source: products
        process:
          productName: productName
field_products:
    plugin: flatten
    source: '@_products'
_hwIds:
    plugin: sub_process
    source: hardwares
    process:
      hwId: hwId
field_hardware_ids:
    plugin: flatten
    source: '@_hwIds'


In order to get to the 2nd level of my data array, I needed to nest the sub_process plugin. I flatten the result afterwards in order to get a numerically indexed array which resembles the multiple value field structure.