SSD functionality questions

Because disks are so slow (compared to the CPU chip), most programs try to load everything they need into memory so that they can access it much faster. Once a program is loaded, it often doesn't need to use the disk any more. In that case, a fast disk will help with load time, but that's about it. Examples of this are programs such as word processors, spreadsheets, photo editors, etc.

Other programs may need to access the disk as they run. The more a program needs to do this, bigger performance improvement you'll see if you put the files it's accessing on an SSD. Typical examples of this would be video editing or batch processing changes to of a folder full of photos.

As far as games go, it depends entirely on the game. You can't really make generalizations - some games load everything up at startup, and other games may load additional data as scenes change or as new characters or objects come onto the screen. The speed-up you'd see with an SSD (assuming the necessary files are ON the SSD) depends on whether the game does this and how often during gameplay you trigger something that requires data from the drive.