Windows 10: Something is taking up over 100 out of over 200 gigs in my downloaded apps on my C drive

So, i don't know what, but something is taking up over 100 gigabytes in my C drive, and i dont know what it is. I have a 512 GB SSD drive, with most of it being on the C drive, however it feels like i only have 150 because of this issue. Here's a screenshot.

:)

 

Windows 10, Download, Disk Space Inadequate, Drive C is SSD, Drive D Has Ample Space, How Direct Downloader to Drive D

When I am redirected to Drives to clean up files, the Downloader lists both drives for cleanup. Although both drives have been cleaned, the downloader still reports inadequate disk space. Drive C only has about 10 Gigs free but Drive D has more than 100.

 

Sudden loss of 165 gig of hard drive space

For no apparent reason I went from more than 165 gig open on my C drive to only 200 mb. The User folder in Windows 10 now shows it takes up more than 165 gig but its contents do not. Help, what should I do?

 

How do I extend my C drive?

I have two partitions in my laptop - C (100 gigs) & D (400 gigs).
Can I use Win 10 disk management to 'shrink volume' in D say 200 gigs and then use extend volume in C to add the available space to C drive? Do I need to backup my existing data in D and C before performing the shrink/extend volume operation?