With SharePoint Framework Extensions, you can customize more facets of the SharePoint experience, including notification areas, toolbars, and list data views.
3 types of SPFx extensions.
Application Customizers allow you to change fixed areas on a page or inject logic using JavaScript. Application Customizers provide access to well-known locations on SharePoint pages that you can modify based on your business and functional requirements.
Field Customizers allow you to control the rendering of any field in a SharePoint List.
CommandSets allow you to put custom commands in the toolbar.