Adding, replacing, and hiding buttons on page layouts
Key Takeaway
To replace a standard button, define the custom button first, then edit the page layout to hide the standard button and display the custom one in its place. Hiding a button changes what the page shows, not what users are allowed to do.
Page layouts control which buttons users see on a record detail page and its related lists. An app builder can edit a page layout to add custom buttons, hide standard buttons, and replace a standard button with a custom one.
What editing a page layout controls
Editing a page layout lets you control which standard and custom buttons are displayed and the order the custom buttons appear in. However, you can't relabel or relocate standard buttons on a record detail page.
You can edit buttons on a page layout in Salesforce Classic and Lightning Experience, in all editions. You need the "Customize Application" permission.