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

No Mac Studio models provide PCIe slots.

However, external expansion is available using high performance Thunderbolt 4 or Thunderbolt 5 ports (USB-C shaped connector).

Early Mac Studio models with M1 Max/M1 Ultra and M2 Max/M2 Ultra processors have Thunderbolt 4 ports (up to 40 Gb/s) whereas later Mac Studio with M3 Ultra/M4 Max and M5 Max/M5 Ultra processors -- including all current options -- have even faster Thunderbolt 5 ports (up to 120 Gb/s).

Thunderbolt 5 is the best and fastest option for recent Mac Studio models to add external storage (and is backwards compatible with older Mac Studio models, as well, albeit operating at slower speeds). External expansion is not limited just to storage, though.

Another great option for external upgrades is to add a PCIe expansion chassis like the affordable OWC Mercury Helios 5S from site sponsor Other World Computing.

Affordable Mac Studio PCIe Expansion Chassis

The OWC Mercury Helios 5S is compatible with all other Apple Silicon Macs with a Thunderbolt 3 port or faster, but the Mac Studio is perhaps the Mac most commonly used by professionals.

Photo Credit: OWC Mercury Helios 5S (External)

Accordingly, for the type of user who back in the day would add a PCIe card to their Mac Pro tower, the OWC Mercury Helios 5S makes it easy to add a bit of extra capability to the Mac Studio, too.

Photo Credit: OWC Mercury Helios 5S (Internal)

Commonly used for high speed storage, 8K video capture, audio DSP, and high speed networking, the OWC Mercury Helios 5S supports a single PCIe 4.0 x4 (x16 mechanical slot) card and provides up to 6000 MB/s bandwidth for simultaneous read/write.

It also has three Thunderbolt 5 ports on the back, each capable of 80 Gb/s of throughput, and even ships with an included Thunderbolt 5 cable.

Ultimately, if you need to add a PCIe card to your Mac Studio, the OWC Mercury Helios 5S is a good option.