Can I upgrade the Mac Mini (2018) memory

If you are asking about memory (RAM) it is not flash memory, but yes an Apple Store or authorized Apple Service Provider can install more. Read Upgrade or install memory in your Mac mini - Apple Support.

Before you do that be sure you actually need more. If your Mac's performance is not being constrained by available memory then adding more will not be beneficial.

You can upgrade the RAM to 32GB, but the 2018 Mac mini has to come apart to upgrade.

No you can not upgrade the 2018 Mac mini’s internal storage to a larger storage drive.

If you are getting low on storage space, then you can move your Music, Photos or Movie Libraries to an external storage drive.

see > Free up storage space - Apple Support

You can upgrade the RAM to 64 GB on that Mac, although it is very unlikely that you would need that much.