These are usually taken for entry into Year 7 or Year 9 ( occasionally Year 8). These take many forms. Some school’s exams are extremely rigorous. Others are easier. Some make past papers freely available - some don’t.
Most Independent schools require applicants to sit an entrance test and also to have an interview. Sometimes the test is written by the school or the school may use a standard test set by the IndependentSchools Examination Board (ISEB), called the Common Entrance Exam.