Massive multiple-input multiple-output (M-MIMO) underpins 5G and is central to beyond-5G/6G systems. By equipping base stations with large antenna arrays, M-MIMO enables aggressive spatial multiplexing that improves spectral and energy efficiency while increasing capacity. This survey synthesizes principles and developments \emph{with an emphasis on advances from 2021--2025}, covering antenna/RF advances, hybrid beamforming, signal processing, and deployment at scale. We compare beamforming strategies and \emph{quantify representative SE/EE trade-offs under stated power models}. We also discuss cell-free architectures, RIS integration, and THz/near-field operation, highlighting open challenges and future directions for sustainable, high-performance 6G networks.
