How to search against a specific field/s, and not every field related to an element?
You can just limit your search to those specific user fields.
It should go something like:
{% set users = craft.users()
    .search('fullName:' ~ query ~ ' OR username:' ~ query)
    .group('creatives')
    .all() %}

More info on searching here: https://docs.craftcms.com/v3/searching.html#supported-syntaxes
Top tip: You can test your search query directly in the search bar within Craft's admin. e.g. See if fullName:Tom OR username:Tom pulls the intended results.