Where are your personal files being stored, on the SSD or on the HDD? Even if apps are installed on the HDD, are they saving their temp and output files on the SSD? Things like  email in/out boxes, favorites, etc can use a lot of disk space and are best stored on the HDD. Have you set Win 10 to move default folders to the HDD? NOTE: You may have to manually copy personal files already stored on the SSD into the correct folder(s) on the HDD, once you move the default folders. And if you're using Windows Defender, a recent bug causes it to eat up lots and lots of C: drive storage space, so read this about fixing that too.
I own a Dell Inspiron 3670 that has a M.2 128 SSD (Toshiba), a 1 TB SATA HDD (Segate), 12 GB Ram, and has Win 10 OS. Recently I have found that I only have ~5 GB of space left on the SSD of the 104 GB available. I am trying to determine how and where the bulk of the SSD space is being used. I took a snapshot of the C: drive to try and determine the space allocation of the directories using Properties. This showed that with all the 17 directories (hidden & not hidden) including Windows 10, only accounts for 50 GB at best.  There are only two apps (programs) that only occupy roughly 2 GBs on the SSD. The majority of apps / programs are on the 1 TB HDD. I have found that the SSD has 5 partitions. However, I am not sure how these play into the issue of only ~5GB remaining.  I will appreciate any thoughts or comments on this issue and how I can regain storage space.
Inspiron 3670 Storage Space