Merge String List by Step

1 Create a new Flow.

2 In the step toolbox, expand Data > Text category

3  Add the Merge String Lists by Line step into the workspace.

4 Define the inputs for the two lists. This example uses two Constant lists.

5 Define the separator value

6 Debug and run the Flow. The output will display the list values combined into a single list.

from: https://documentation.decisions.com/preview/v9/en/cc505bb5-3a71-449c-a5bf-be20829324ad/1