Mac Mini M4 storage upgrade

Because the M4 Mac mini does not have a conventional NVMe SSD.

There are 512GB, 1T and 2T upgrade options available at the time of purchase.

see > Buy Mac mini - Apple

The other option, is to move your large Photos, Music or Movie Libraries to external storage.

Move your Photos library to save space on your Mac - Apple Support

Change where your music files are stored on Mac - Apple Support

Move your iMovie for Mac library - Apple Support

It is what it is, vent here Feedback - Mac mini - Apple (if you must).

Macs (iMacs and Mac minis) have not been user upgradeable for many years now. Its time to move on or you can let Apple know how you feel in www.apple.com/feedback.