When you do an install of Windows, there are various options as to how the disk will be partitioned. The default will be to use the whole disk for C:. Microsoft will pinch some space, the 100 MB (not mb - MB is 8 billion times the size of mb), which Microsoft needs as part of its boot up and recovery processes. As it is less than 0.1% of the disk space, then it is not wasting any significant amount of space.
What size disk does the computer have? My guess is that it is nominally a 200 GB drive, but Microsoft have not got their corporate heads around using the correct units. When they report 189 GB (not gb), they should be saying 189 GiB.
1 GB is 10⁹ bytes. 1 GiB is 2³⁰ bytes = 1.074 GB.
In Disk Management, right click on the graphic showing C: and see if it has the option t shrink the partition. If it has, shrink it and this should release some of the drive as unallocated space. Next right click on this unallocated space and make a New Simple Partition, which can be D:. Don't make C: too small.
When you have D:, open Windows/File Explorer, and right click on your personal folders (such as Documents, Music, Pictures and Videos), go into properties and look for the location tab. You can get Windows to move the folders and their contents from C: to D:. I keep C: for Windows and application programs, and D: for all my data.
I hope this helps.