Bad news, SSD is soldered (as is the CPU). RAM is super easy to swap out, however:
https://www.engadget.com/2018/11/09/mac-mini-2018-teardown/
I don't understand the thinking here. I can understand the CPU, but the SSD? I mean, SSD's fail all the time. They've gotten more reliable in recent years, but unlike mechanical hard drives that often slowly fail & allow you to get your data off first, SSD's typically just flat-out die. I don't understand why they wouldn't just stick a simple NVMe slot in there, especially because if your SSD dies, then they have to replace your entire motherboard under the warranty or AppleCare. The soldered storage is in highlighted in orange (bottom right) in the picture below: (128GB Toshiba TSB3225V81199TWNA1 flash storage)
Granted, it does say that the new SSD chip is PCIe-based, and that you can get up to 3.4GB/s sequential read speed, which puts it nearly on-par with the advertised speeds of the Samsung 960 PRO NVMe SSD chip (3.5GB/s max), so outside of the even faster (and vastly more expensive) ones I install for business clients, like Kingston's DCP1000 (four NVMe chips on a single card) or Fusion-io's (now Sandisk, I guess) super-ridiculous cards, that's pretty dang speedy. I'd really love to get my hands on the new Mini, which is basically a SoC now (for better or for worse), because I bet that little sucker
flies due to having everything interconnected like that.