I've written software for a few years to earn a crust. Nearly all Management Info stuff so no safety critical risk of any form.
I have no formal qualifications just, on the job training and experience, which may not be adequate or relevant to my next job.
My brother retrained to become an electrician later in life. He is regulalry tested certified and the scope of work he can do is constrained by his training.
I could be hired to help write the software for a piece of medical equipment that could kill someone if the software is wrong. But legally only someone like my brother could wire up the plug in the hospital..