Magento\Config\Block\System\Config\Edit::_prepareLayout()
The button’s initial HTML
<button
class="action-default scalable save primary"
data-mage-init="{"button":{"event":"save","target":"#config-edit-form"}}"
data-ui-id="page-actions-toolbar-save-button"
id="save"
title="Save Config"
type="button"
><span>Save Config</span></button>
The button’s final HTML
<button
class="action-default scalable save primary ui-button ui-widget ui-state-default ui-corner-all ui-button-text-only"
data-ui-id="page-actions-toolbar-save-button" role="button" aria-disabled="false"
id="save"
title="Save Config"
type="button"
><span class="ui-button-text"><span>Save Config</span></span></button>
lib/web/mage/backend/button::_click()