your "user" files/folders, such as desktop files, document files, music, pictures, videos, etc... tend to be placed on C drive. many programs like to automatically create personalized folders and place files into those folders, which again would all be on c drive.
ugly and painful, right click and goto properties for folders on C drive. to see how much a give folder is taken up space, keep doing it till you find a program or videos, or pictures or what not that is eating up the space.
you can also do a "search" via windows explorer on c drive and let it hit all the files, and then reorder files based on "size" to see the larger files, then look at folder paths to see were these files are located for the larger size files. this is slow and painful but might identify some programs or like.
if you can identify programs, then you might be able to uninstall the programs, then reinstall the programs via a custom install to move the programs to another drive.
one drive, dropbox, google drive, etc... some times they like to store files on C drive. if you do not configure them correctly for how they sync files.
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not sure what the recommended keep below on SSD (solid state drives are) assuming 100gb C drive is a SSD drive. generally on older regular HDD drives 70% and below you want to keep below in how full you fill them. once you go over a given number the performance takes a pretty big hit, in both reading and writing.
i have not done a "custom install" for windows 10, but i would imagine that would be more ideal way to shove, program files, program files(x86), user, folders over to another drive. but doing this can cause some problems with various programs.
best bet, would be upgrade your C drive to a larger size. 100gb, just not much. 1T / 1,000GB would be much better. 512GB would even suffice.
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