Cascading delete operations occur when a record that has a master-detail relationship with another record is deleted, and the child records are also deleted as a result. This action may cause triggers to fire on both the parent and the child records, depending on the trigger definition and the operation type. For example, if a trigger is defined on the child object to execute after delete, it will fire when the parent record is deleted and the child records are cascaded. Similarly, if a trigger is defined on the parent object to execute before delete, it will fire when the parent record is deleted and before the child records are cascaded. However, updates to feed items, renaming or replacing a picklist entry, and changing a user's default division do not directly cause triggers to fire, unless they are part of a larger transaction that involves a DML operation on a trigger-enabled object. References: Triggers and Merge Statements, Triggers and Recovered Records, Triggers

A lightning component is a reusable unit of code that can be used to override the standard actions of an object, such as Edit, in Lightning Experience and the Salesforce mobile app1. A lightning component can provide a custom user interface and logic for the action, and can interact with the record data using the force:recordData component2. A lightning action, a lightning page, and a lightning flow are not valid options for overriding the standard Edit button for an object. References:
* Standard Actions and Overrides Basics
* Override Standard Actions with Aura Components

* The @testSetup annotation is a way to create test data for a test class. It allows you to define a method that runs once before any test methods in the class and inserts common test data into the test context12.
* The @testSetup annotation is not supported when the GisTest(SeeAllData=True) annotation is used.
The GisTest(SeeAllData=True) annotation allows a test class or method to access all data in the organization, including pre-existing data that the test didn't create. If this annotation is used, the test setup method is skipped and no test data is inserted12.
* Test data is inserted once for all test methods in a class when the @testSetup annotation is used. This means that the test data is shared across all test methods and only counts once towards the governor limits. This can improve the performance and efficiency of the test class12.
* A method defined with the @testSetup annotation does not execute once for each test method in the test class and does not count towards system limits. It only executes once before any test methods and only counts once towards the governor limits. However, any DML operations or queries performed in the individual test methods do count towards the system limits12.
References:
* 1: Using Test Setup Methods
* 2: Cert Prep: Platform Developer I: Testing and Debugging