Is the SSD replaceable

Technically these are not true SSD’s, think how the SSD has two parts, the Flash Chips and the microcontroller which manages the data either being written or read from the Flash chips cells, as well as the cleanup. This microcontroller function is within the systems SoC chip, these modules are just Raw Flash (NAND) chips.

You’re in luck! There is a company who makes replacement Flash modules. At this time they only offer the M1 system modules Mac Studio M1 / Ultra Hard Drive Upgrade. I’m sure within the year the 2025 M3/M4 systems will be produced either as a bare board (you need to install the virgin NAND chips) or complete modules. Do keep in mind you’ll also need a second Mac to prep the Flash module/s so you can then install a fresh macOS and then restore your stuff from a backup drive. The original modules can’t be reused as they are locked to your system.

Here’s a bit more Upgrade experiments & Mac Studio Storage Upgrade Success