Many professional signs are also made from bent sheetmetal… Rather than woodworking tools - there are a whole set of tools (from hand tools to jigs and machinery) use to form metal channel letters.
There are also lots of tools to cut, bevel and notch different foam board. Some like those from Logan use knife blades, others use hot wire cutters or hot knives and still others use saw blades. When EIFS was all the rage in our area, we became rather proficient in cutting and bevelling foam for exterior house “siding” having the appearance of masonry with additional architectural detail.
But your thread started with the notion of creating plywood letters to apply to a plywood backer board. Like Anton - I questioned using plywood for the letters, suggesting MDF or MDO - which can be cut and machined a bit like wood - but does not have voids like many grades of plywood - and is a lot cheaper than the Baltic Birch plywood that I also mentioned.
Wood signs can also be created in relief by routing the letters into a board. There are templates, router bits and router guides of varying quality and utility to help with this. Then painting or even gold-leafing the inset letters can produce a dramatic sign.