New M4 mini has replaceable/upgradeable 2 NAND SSD.

If these are like the SSDs in the Mac Studios and the M2 Ultra Mac Pro,

• They are not standard, self-contained SSDs – but raw flash modules. Apple might be the only source for them (through the Self Service Repair program). If you look at Apple's pricing for flash modules for the M2 Ultra Mac Pro, you will see they are not cheap ($1000 for 2 TB, $1600 for 4 TB, $2800 for 8 TB).
• They might be buried deep within the machine, like they are on the Mac Studio.
• They need to be cryptographically paired with the Apple Silicon processor – which acts as a SSD controller and does real-time encryption/decryption using keys held within its Secure Enclave. Change out the modules - and you will probably need the aid of a second Mac running Apple Configurator just to get the Mini back to the state where you can re-install macOS, and restore your data from your backups (you did make those, right?).

The applicable word here is "repairable" (if that), not "upgradable". Don't plan on being able to cheaply upgrade internal storage after you buy the Mac mini, because it is not going to happen. Adding external drives will still be your only practical option for getting more storage "after the fact."

Replaceable is an academic term here as while under warranty, it would void the warranty and a DIY operation has considerable opportunity to permanently damage the mini. OWC now has Thunderbolt 5 Envoy Ultra external drives capable of 6000 MB/s. Zero risk compared to M4 mini surgery.

The MacStudio and the MacPro has "replaceable" SSDs but as many have found that you can't upgrade it even with an Apple SSD from one to another. So, if no one has created 3rd party upgrades for these don't expect any for the Mini as Apple is likely using the same parts.

However, it does mean that "SSD write angst" may no longer be an issue as Apple may be able to simply replace the SSD, reflash the logic board somehow instead of having to replace the entire logic board.

FWIW, that is replacing the chips on the same board shipped with the Mac and not a simple 3rd party board. All of which will automatically void any warranty. Besides, only a handful of people with the skill set and equipment will be able to do it. Also, I notice is that the person in the video is taking next to no anti-static precautions which from past experience is not a great idea as you can spend a lot of work and end up with trash.

To note there are additional small chips on the board one of which is likely some form of ID/Serial number which Apple has likely locked to the Mini like the do with many other components on other products which would preclude the use of a third party "complete" modules.

I would not try it myself and I have done many Mac tear downs. One slip up and you have a brick.