Assign a Custom Record Page to Lightning Apps, or Make It the Default for All

When you activate a custom record page in the Lightning App Builder, you now have two options. You can make the record page the default for all your users. Or you can assign the record page to one or more Lightning apps to give your users access to a page customized for the app that they’re working in.

Use the new Assignments by App column in the Lightning Record Pages related list in the Object Manager to see which Lightning apps each record page is assigned to.