I am using Windows 10 on an HP desktop computer with 16 Meg of RAM and the newest version of Photoshop. I am wondering – is Photoshop able to use all of the RAM available?
If you are running Windows 10 64bit, and a 64bit version of Photoshop, then: yes (minus the amount already in use for other things such as the OS and other background processes, Intel graphics ram usage, etc.).
You set memory allocation in Photoshop preferences - but for the reasons mentioned above, you should never set it to the full available amount. That will choke the rest of the system. Always leave 2 - 4 GB, more if you routinely run other resource-hungry applications simultaneously.