adding custom objects to lightning app builder

You can create and modify restriction rules using the Tooling or Metadata API. How: For information on enabling this feature, contact Salesforce. Restriction rules let you configure truly private access for these objects. For custom objects, users can see all detail records. Why: Without restriction rules, users with access to an account can see its contracts, tasks, and events, even when the organization-wide default is set to Private. They provide another layer of access control on top of your existing sharing settings and let you configure truly private visibility. You can create and manage restriction rules via the Tooling and Metadata APIs. Restriction rules are available for custom objects, contracts, tasks, and events.

Create restriction rules to control which subset of records you allow specified groups of users to see. Secure your data and boost productivity by permitting your users to see only the records necessary for their job function. Below is my top pick from Summer '21 release from App Builder & Development perspective: - Control Access to Sensitive Data with Restriction Rules (Beta)

Adding custom objects to lightning app builder