Content:
The external variable data in the variable update index needs to be "imported" to the respective process instances in the process instance indices. 

Note: external variable data will only be applied to "normal" process instances, event based process instances will not receive external variable updates (for now at least).

Also see notes here: https://confluence.camunda.com/display/CO/2021-08-20+External+Variable+API+Summary?src=jira 

AT:

• External variable data is written to their respective process instances, identified via the processInstanceId in the variable data
• This import can handle rolled over indices as well (should also add IT for the scenario that indices we read from have already been rolled over)