According to the Integration Procedure Actions page, a Remote Action can use a function in an Additional Output value field or in a Remote Options value field. According to the Response Actions page, a Response Action can use a function in an Additional Input value field.

The FirstName, LastName, and BirthDate fields should be placed inside the Type Ahead Block, so that they can receive the data from the selected record in the Type Ahead list. The Type Ahead Block has a Data JSON Path property that specifies the path to the data node that contains the record fields. For example, if the Data JSON Path is set to Contact, then the FirstName field should have a Value property set to {{Contact.
FirstName}}.

In Preview, the JSON node that shows the data sent to the Response Action is the name of the Response Action itself. In this case, it is ResponseCart. The other nodes show either the input data or the output data of the Integration Procedure.

In OmniStudio Integration Procedures, when you want to reference the output of an element in a nested block (e.g., for use in a Remote Action or another step), you must use dot notation that mirrors the block structure hierarchy.
In the exhibit:
* Action1 is nested inside:
* BlockB, which is nested inside:
* BlockA
Therefore, the correct Send JSON Path to access the result of Action1 is:
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BlockA.BlockB.Action1
This format complies with the standard output reference syntax in Integration Procedures, which follows the pattern:
php-template
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<ParentBlock>.<ChildBlock>.<ActionElement>
Incorrect Options:
* B. BlockA:BlockB:Action1 - Uses colons instead of dots. This is invalid syntax in JSON path resolution within Integration Procedures.
* C. Action1:BlockB:BlockA - Incorrect order and invalid separators.
* D. Action1.BlockB.BlockA - Incorrect order (reversed path).
Reference:
Salesforce OmniStudio Developer Guide # Integration Procedure Element JSON Pathing Best Practices for Send JSON Path # Nested Blocks and Element Referencing

According to the FlexCard Designer Overview page, another possible cause of a field showing only the label is that "the field is null on the Salesforce record".