Apple says that the flash storage is indeed affixed to the logic board and cannot be removed. 

However, for the 4TB and 8TB configurations, Apple says that a flash storage expansion board is attached to a connector on the logic board. 

In the 256GB, 512GB, 1TB, and 2TB configurations, the expansion board and connector are not present.

Apple adds that the flash storage and logic board are paired together for hardware encryption, so data will be lost if the logic board is replaced. 

For this reason, it is recommended that customers back up their files on a regular basis.

Update

 the 256 GB configurations : 

there are 2 flash nand soldered, 1 on the back side of the logic board, 1 on the front side

 the 512GB to 2 TB : 

there are 4 flash nand soldered, 2 on the back side of the logic board, 2 on the front side

the 4TB and 8 TB configurations :

yes there is a flash expansion module, but it contains only HALF of the flash storage

the other half of storage is still soldered to the logic board.

So for this iMac 27” 5K 2020 :

all configs from 256GB to 2TB of storage have 100% of their storage soldered

the 4 TB configuration has 2TB soldered and 2 TB on a separate module 

similar to the iMac Pro and Mac Pro

the 8 TB configuraiton has 4TB soldered and 4 TB on a separate module

This iMac has been conceived with limited engineering efforts, 

so even if there is plenty of space, Apple choose to not offer upgradability