7 is actually a catch-all for anything that's not 1-6 that are actually defined. Often 7 is used for PLA because it's common that way but it's not wrong if it's used for other polymers.

Your plastic bags are probably not 6 or 7, my guess is that they are recyclable material. They are probably just challenging to recycle because they fly all over the place when you collect them. I think this is basically why stores collect the film stuff, it's too physically annoying to handle it otherwise.

I think I see 7 mostly on things like to-go containers, cups, and lids. Of course straws are PET I think but they aren't numbered so they're always trash in practice.