Dear  Dorothy Fan‌,

1- The lookup to UserName Field will get the data to the control in the following format: 
ID of the item in the Reviewer Lists +";#"+ User Display Name

2- "ID conncted to" and "Text connected to" cannot be linked to Person or Group field. Indeed they are connected to single line of text field.

3- You can use the display name in sending notification and setting items permissions in Nintex Workflow since the actions can resolve the display name and retrieve implicitly the related user

Cheers...

Hi ‌,

As an alternative, you can create a workflow which uses the ID of the selected item as mentioned by ‌ in option 2, to use in a 'Query list' action to extract the user/group from 'Reviewer Lists' and then use the 'Update item' action to update the user/group in the item.

Cheers,

Rick