This cinderblock outbuilding's windows and doors were covered by metal siding. How can I turn them back into windows and doors?
Marking the edges of the siding and use a circular saw with a high tooth count carbide blade. I have cut hundreds of feet of metal both ferrous and aluminum with a circular saw. The hard part here will be extending your openings and making wider looking walls but it can be done. Best to pull all the trim so you have the max opening and make 1 cut, put new trim (jambs and sills) then a finish piece around to cover.
A tip, once I pull the trim inside, I drill holes at the corners then I cut the opening 1/2” wider than the holes. The trim fits easily in the hole and the exterior trim around covers the small gap.