A Fusion Drive is a special setup of two drives a standard drive and a small SSD. When joined the SSD is no longer seen as its only purpose is to cache more heavily used files from the HDD. So basically the OS copies off the files from the HDD and places them on to the SSD where they are more quickly accessed.
A SSHD hybrid drive is very similar unlike the Fusion Drive the task of moving files from the slower HD to the SSD is all done within the single drive!
OK to upgrade your drive you need to first break the Fusion Drive set following this guide: How to split up a Fusion Drive at that point you can disassemble your mini to replace the HDD with the bigger SSHD following this IFIXIT guide: Mac mini Late 2014のハードディスクドライブの交換