Gretchen, most of it is from searching and reading -
I've learned a lot by reading old online sewing machine manuals and sewing instruction books put out by the manufacturer. They are all over the place - Needlebar has some, Ismacs has some, and I've bought a lot of them too. I've always found bits and pieces and the fun part is starting to put them together and see the pattern that emergize.
Like this particular foot. I got it with a machine and looked up the part number. I've since learned that there is an older back clamp and V2 version, but this is the very first time I've come across anythink about there being a back clamp version.