Since 2020, Lenovo has organized the ThinkCentre M Series primarily into standard tiering (M70, M80, M90) and released them in chronological generations (Gen 1 through Gen 5).
To give you the most accurate overview, the configurations are separated by form factor because Tiny (1L micro PCs) use laptop-style memory and have limited space for GPUs, whereas SFF (Small Form Factor) and Tower (t) models have PCIe slots for standalone desktop GPUs.
The Gen 1 lineup established the foundational memory limits for the modern M-series, utilizing DDR4 RAM.