1) Circular depressions indicate spot welds
2) Cuts are made between welds using a cut-off wheel. Not necessary or even desirable to cut through top panel.
3) Cold chisel is driven between panels, tearing the metal at the 'score. '
4) Edges are folded up around weld. These are be with vise grips and twisted as shown.
5) Fragment tears off clean, leaving only small pimple to be ground off.