The pedal came off the non drive side (left) crank arm because it is modern and compatible with your pedals. The drive side original crank arm is likely French and the pedal you forced on did not have matching threads. Also that replaced crank arm does not match the spindle axle thing and goes on too far. Stick with it. We learn as we go. A PX-10 is a nice place to start. You will have a very nice rider eventually.
link to other thread with a picture:
https://www.bikeforums.net/classic-v...d-pueugot.html
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I don't do: disks, tubeless, e-shifting, or bead head nymphs.