I have machined over 1,000 CZ parts, mostly internal, but also many frames and slides.
Frames are cast, but the steel "works" like a forging. I have never seen a casting void in a CZ frame. CZ does not heat treat the frame, so it has to have a high carbon content, which is tough to machine since it dulls cutters fairly quickly. Hence tool marks. No heat treating and a high carbon content creates a durable, yet ductile frame. This is one of the reasons I hold CZ's in such high regard.
The slides are even harder than the frames, and CZ does heat treat the breech face, which extends back to the rear sight dovetail. You cannot use HSS cutters here, carbide only. A CZ will never peen or burr it's firing pin hole. Ever.
I'm no metallurgist, just a machinist, but CZ is a hold out in world of compromise that has gone to plastic, MIM, and low grade castings. There are many areas CZ could compromise, maybe one day they will, but for now, they offer incredible value for the money.
In reality a Sig should cost what a CZ does, and vice versa (not a jab a Sig).