I am not a Windows person (I use Linux), and you did not mention if your computer was a desktop or laptop model. If it is a laptop, and the solutions suggested by all of the above (and they all seem solid advice to me) don't work, you have alternatives. Presumably, your SSD is an NVMe drive. You can switch this out quite easily yourself (there are computer model specific YouTube videos on how to do this). Most after market drives come with the necessary software to make the switch effortless. With my last laptop purchase, I immediately replaced the 250 GB drive with a 1 TB drive. I also installed a 1 TB 2.5" SATA SSD. 1 TB drives are available at Amazon for under $100. You can get 2 TB Sabrent Rocket NVMe for under $200 (I like the Sabrent Rocket and the SK Hynix NVMes.) Additionally, with SSDs, the larger the capacity, the longer the life expectancy of the drive, and the greater the speed, in most cases. Also, beware of NVMes with a "Q" designation, variation of it (QLC). They are usually less expensive, but have shorter life expectancy, and are slower. Lastly, it is worth remembering that files stored on an HDD can be retrieved if the drive fails. Most of the time. However, if an SSD fails, they're gone. Most of the time.
Also, there are ways to find what is being stored, and where it is located. In Windows 10, go to Settings>System>Storage. First, you can see the capacity of your C drive. If it is less than the SSD, you have a partition, which might explain some of this issue. Second, under the C drive listing, there are typically three categories listed. Below them is a line See More Categories. Click on that and you should be able to see where your storage issue lies.