The TestDataFactory class is a special type of class—it is a public class that is annotated with isTest and can be accessed only from a running test. Test utility classes contain methods that can be called by test methods to perform useful tasks, such as setting up test data. Test utility classes are excluded from the org’s code size limit.
Using the isTest(SeeAllData=True) Annotation - https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_testing_seealldata_using.htm
Create Test Data for Apex Tests (Trailhead) - https://trailhead.salesforce.com/en/content/learn/modules/apex_testing/apex_testing_data