How to preprocess view field?
The output of views field templates is flat markup. You can't change a render array like you can in field templates when rendering entities in view modes.

So if you want to change the output you have to start from scratch with the values that are in the database. In views field templates you can use the getValue() method from the field variable. As parameter you have to provide row:

{{ field.value(row) }}


The result is a single value or an array for multivalue fields.

Example

views-view-field--created.html.twig

The raw value from created is a unix timestamp

Timestamp: {{ field.value(row) }}


You can use the timestamp for example to reformat the date with the date filter from twig:

Date: {{ field.value(row)|date("m/d/Y") }}