I think you may be confusing or merging the term Frames with "Foundations"...Often, if not always?...Plastic frames come with plastic foundation as an all in one unit. However, many people (including me) use wooden frames. At that point you have basically three options, insert plastic foundation into that frame, or wax foundation, or no foundation (foundationless).
The bees will use either the plastic or wax foundation as a starting point/guide to draw comb...Or, simply the top, or starter strip of an empty frame in the case of foundationless to draw the comb.
Hope this helps! :)