Sounds like you're confusing extenders (aka teleconverters) with extension tubes.
Extenders multiply focal length, and contain lens elements (meaning you lose some IQ). Canon makes 1.4x and 2x, and they cost you 1 or 2 stops of light, respectively. Those do multiply the max mag as well, so for example, the 70-200/2.8 with a 2x becomes a 140-400mm f/5.6 with a max mag of 0.42x.
Extension tubes contain no optics, just air. They move the lens further from the sensor, which 1) shortens the MFD, 2) shortens the max FD, i.e. you can't focus to infinity, and 3) costs you a bit of light (but not much). Extension tubes are used with wide and normal lenses, and by shortening the MFD they allow closer focus and higher mag. They're a lot cheaper than an extender, and unlike extenders where 3rd party brands are optically inferior to the Canon, 3rd party extenders are no different (to paraphrase Bryan at TDP, Canon air is no better than Kenko air...but you pay more for the Canon air!).