How can I cut a hole through metal siding to open up a doorway or window?
I'd go with offset aviation snips to cut the sheet metal either start where sheets overlap or drill a half-inch pilot hole  4&quot; from the edge and spiral out from that.
Sheet metal has sharp edges wear work gloves.
A sabre saw would also work but may scratch the surface finish. sawing sheet metal is VERY NOISY.
An angle grinder will make an even worse racket and burn the paint near the edge leading to accelerated rusting.
Straight snips will not work on the sheets while they are in place, but if you remove them they can be cut, but it's tricky, you probably want a helper spreading the cut.