What custom processor options are available for the Mac Studio? Is it possible to upgrade the processor after purchase?

As confirmed by site sponsor OWC in its teardown, the initial "M1 2022" Mac Studio models do not have a processor (SoC) that is designed to be upgraded after purchase. Subsequent Mac Studio models are the same general design and likewise do not have upgradable processors.

Photo Credit: Other World Computing (Mac Studio Processor)

However, at the time of initial system purchase, the processor can, or could, be custom configured with a faster option.

For the original "2022 M1" models, the entry-level Mac Studio "M1 Max" 10 CPU/24 GPU could be upgraded to a M1 Max 10 CPU/32 GPU, M1 Ultra 20 CPU/48 GPU, or M1 Ultra 20 CPU/64 GPU processor whereas the higher-end Mac Studio "M1 Ultra" 20 CPU/48 GPU could be upgraded to a M1 Ultra 20 CPU/64 GPU processor.

For the subsequent "2023 M2" models, the entry-level Mac Studio "M2 Max" 12 CPU/30 GPU can be upgraded to an M2 Max 12 CPU/38 GPU, M2 Ultra 24 CPU/60 GPU, or M2 Ultra 24 CPU/76 GPU processor whereas the higher-end Mac Studio "M2 Ultra" 24 CPU/60 GPU can be upgraded to an M2 Ultra 24 CPU/76 GPU processor.

Finally, for the current "2025 M3/M4" Mac Studio models -- the Mac Studio "M3 Ultra" 28 CPU/60 GPU (the higher-end model) -- can be upgraded to a M3 Ultra 32 CPU/80 GPU processor whereas the Mac Studio "M4 Max" 14 CPU/32 GPU (the entry-level model), can be upgraded to a M4 Max 16 CPU/40 GPU processor.

What type of RAM does the Mac Studio use? Can it be upgraded?

The Mac Studio models all have "unified" RAM onboard that is integrated with the processor (SoC). The RAM cannot be upgraded after the initial system purchase, but all models can, or could, be equipped at the time of initial system purchase with more RAM than its default configuration, though:

To reiterate, no Mac Studio models have RAM that can be upgraded after the initial purchase of the system.

Does the Mac Studio provide PCIe slots? What external expansion is available?

No Mac Studio models provide PCIe slots. The highest performance external expansion option is provided by Thunderbolt 4 or Thunderbolt 5 ports (USB-C shaped connector).

Models equipped with an M1 Max or M2 Max processor have four Thunderbolt 4 ports and models equipped with an M1 Ultra or M2 Ultra processor have six Thunderbolt 4 ports. Likewise, models equipped with an M3 Ultra processor have six Thunderbolt 5 ports and those with an M4 Max processor have four Thunderbolt 5 ports (the M3 Ultra is the higher-end model).