What is the "C: Drive" on Windows?
The C: drive is the "drive" where Windows is installed by default, i.e. it's the drive with the drive letter C:. If a second drive is found, that would normally be the D: drive.
All system files of Windows are stored in the C: drive by default. You'll have a folder with the filepath C:\Windows where they are stored. Physically, the C: drive could be a hard disk drive or a solid state drive. As SSDs are cheaper and more popular today, new PCs may have only a SSD (C:), or a SSD and a HDD, in which case the operating system will be installed in the SSD because it's faster (so C: and D: drive letters respectively). Older PCs only had a HDD (C:).
In other words, the C: drive is just where Windows is installed, regardless of the physical drive is a SSD or a HDD.
C: drive.