getChildRelationships() can be used to retrieve all the related or child objects of an object in Salesforce Apex.

Sample code:

Schema.DescribeSObjectResult objResult 
	= Account.SObjectType.getDescribe();
List < Schema.ChildRelationship > objRelationships 
	= objResult.getChildRelationships();

for ( 
	Schema.ChildRelationship cr : objRelationships 
) {
	
	if ( 
		String.isNotBlank( 
			cr.getRelationshipName() 
		) 
	) {
	
		System.debug(
			cr.getRelationshipName()
		);
	
	}

}

Sub–Query can be used to retrieve child or related records in Salesforce.

SOQL Query:

SELECT Name,
(SELECT Name, AccountId
From Contacts)
FROM Account