To make your muscles hard and dense, you must consistently lift heavier weights or increase your workout volume over time.
To help you get exactly what you need, direct overviews for both interpretations are provided below.
According to the ACE Fitness guide on core training , developing rock-hard abs requires targeting all four major abdominal muscle groups and incorporating back stabilizers.
🏋️ 1. Physical Fitness: Building a "Rock Hard" Physique