For the Mac mini 2014 the memory is as you state soldered to the logic board, as such it is generally regarded as impossible to upgrade the memory. This leaves the only option as being to sell it and buy a better model.
In theory one could unsolder the existing chips and solder new bigger capacity ones, this indeed did use to be commonly done in the olden days. ;) I remember some enterprising people doing this to upgrade the original Macintosh 128KB to a 'Fat Mac' with 512KB, I even had this done to my own one back in 1985.
However newer logic boards, chips and manufacturing techniques mean that these days an ordinary soldering iron is totally useless for this purpose and you need tools equivalent to those used originally to build the logic boards, as such it is not possible unless you are a major manufacturer and they have no interest in doing this. Any DIY attempt would merely destroy either the logic board or chips or both.
The earlier Mac mini models i.e. 2012 and earlier or the newer 2018 model all have socketed memory and can be upgraded it is only the 2014 model that does not.