Hey,
you could do it like this:
Full Flow:

Explanation:
My MS Form:
Just a placeholder field to trigger the flow...

My SharePoint List A:
User is a Person (People Picker) Column, ValueX is a simple Text Column.

My SharePoint List B:
it's empty for the moment

Now the flow:
First we trigger the flow When a new response is submitted and get all the Data using the Get response details action using the Dynamic content Response Id from the trigger.
After that, use a Get items action to get all items from the SharePoint List A.

Then, add an Apply to each action and use the Dynamic content value from the Get items action in the Select an output from previous steps field.

Inside the Apply to each action, add a Condition action.
On the left side, select the Dynamic content User Email from the Get items action.
The operator in the middle should be is equal to.
on the right side, select the Dynamic content Responder's Email from the Get response details action.

In the left If yes section of the Condition action, simply add a Create item action and point it towards List B.
You can then use the Dynamic content ValueX there.

Check
When I submit a Form Response with my Admin User the flow runs successfully:

List B then has a new item with the expected Valuex DEF from List A of the (Adm) User Row:

let us know if this works for you, cheers,
caspar