Ultimately it's somebody's capabilities that make them an Engineer, not a specific route to that point. Similarly, it's somebody's abilility to do a job that results in them being authorised as a signatory, not whether they fit any particular mould of qualifications, experience, etc.
It just happens that a lot of Engineers, and signatories, have Engineering degrees. Many others have completed long and tough apprenticeships or equivalent courses. More importantly, the vast majority of them have years of relevant experience in the job during which they've proved their worth.
There is of-course a third breed, those who build them in the first place. Wherever I've worked, they've been called "craftsmen" or "fitters" but I imagine there are a lot of other job titles around the world.
I understand that the people who built and maintained the Titanic were far better qualified than those who built Noah's arc.
G