How to alter output of markup on a specific views field
Views is mainly a UI tool, also for theming. Check this box in the field:

REWRITE RESULTS
[x] Override the output of this field with custom text

This opens a text box which accepts Twig code and field tokens of other fields which are placed above. Exclude those fields from display if you want to us them only in Twig. You can check the available tokens in REPLACEMENTS PATTERN below the text box. Look in REPLACEMENT PATTERNS for raw field values, don't use formatted output in Twig logic.