When a parent record is deleted, delete all related child records.

trigger AccountTrigger on Account (after delete) {
    if(trigger.isAfter && trigger.isDelete) {
        AccountTriggerHandler.deleteRelatedContacts(trigger.old);
    }
}

public class AccountTriggerHandler {
    public static void deleteRelatedContacts(List<account> oldAccounts) {
        // Collect all deleted accouts Id
        Set<id> accIds = New Set<id>();
        
        for(Account acc: oldAccounts){
            accIds.add(acc.Id);
        }

        if(!accIds.isEmpty()) {
            // Retrieve all related contact
            List<contact> relatedContacts = [SELECT Id FROM Contact WHERE AccountId IN: accIds];
            if(!relatedContacts.isEmpty()){
                Delete relatedContacts;
            }
        }
    }
}

@isTest
private class PracticeAccountTest {
    @isTest
    private static void testDeleteRelatedContacts() {
        Account testAccount1 = new Account(Name = 'Test Account 1');
        insert testAccount1;
        
        Contact testContact1 = new Contact(FirstName = 'John', LastName = 'Doe', AccountId = testAccount1.Id);
        insert testContact1;

        Account testAccount2 = new Account(Name = 'Test Account 2');
        insert testAccount2;

        Contact testContact2 = new Contact(FirstName = 'Jane', LastName = 'Smith', AccountId = testAccount2.Id);
        insert testContact2;
        
        List<account> accountsToDeleteContacts = new List<account>{testAccount1, testAccount2};
        List<contact> contactList = new List<contact>{testContact1, testContact2};
        System.Test.startTest();
        PracticeAccount.deleteRelatedContacts(accountsToDeleteContacts);
        System.Test.stopTest();
        
        List<contact> remainingContacts = [SELECT Id FROM Contact WHERE Id IN :contactList];
        System.assertEquals(0, remainingContacts.size(), 'Related Contacts should be deleted');
    }
}