SharePoint Command bar customization helps personalize a list to suit specific requirements.
In this video you will learn how to use this JSON based feature to allow for basic changes to the command bar, including modification of icon and/or text, hiding existing options or repositioning commands.
Command bar customization syntax reference - https://learn.microsoft.com/en-us/sharepoint/dev/declarative-customization/view-commandbar-formatting
Fluent UI Icons: https://developer.microsoft.com/en-us/fluentui#/styles/web/icons