The practicalities of turning wrought iron into steel, and then hardening that steel by heat treatment, seem to have been worked out long ago in, well, the so-called iron age.
An oft-quoted line from Homer’s Odyssey is:"… when a man who works as a blacksmith plunges a screaming great axe blade or adze into cold water, treating it for temper, since this is the way steel is made strong, ... "
Which, allowing for the modern language of the translation, still suggests that the basic method of hardening and tempering steel was well-known in the 4th century BC. Admittedly however Homer's description does not make it entirely clear whether this was deliberate quench-hardening, rather than simply cooling, nor whether the required temper was achieved by a deliberate second stage tempering heat-treatment, or was achieved by a simpler, one-stage, slower-cooling method, called slack-quenching. The two methods produce greatly different crystal structures, and so subtley different mechanical properties. And of course Homer, as a blind poet, may very well have never seen nor understood, the arcane, almost mystical, metal-working methods he was poetically describing.
But you are correct that these are very sophisticated processing techniques, despite being only known in practical terms and which could only be achieved if one had years of experience and followed the established methods with very little deviation. In the days before it was possible to assess metal quality or accurately measure temperature, such blind following of the age-old ways to the letter was essential, although of course I doubt that many medieval artisans could read or write, and so they couldn't commit their own hard won experience to the written word. Of necessity therefore the father-to-son , 'traditional' way of doing things remained very common in a lot of metal processing, even up until quite recently.
For example, raw smelted copper (blister copper) was traditionally refined by "poling" a method of stirring the liquid copper with poles of green ash wood. This was still routinely done into the 20th century (and it may still continue), although it is now known that it is the hydrocarbons and carbon monoxide released by the wood which burn off the impurities ... and so modern foundries usually use natural gas. Similarly, going back to quench-hardening steel, I've seen old descriptions that state the hot metal has to be quenched into "the urine from a ginger-haired boy". Bizarre though this initially sounds, this instruction is not actually so odd now that the processes involved are well understood: urine has a lower thermal conductivity than water, and would produce a slacker quench (conversely quenching in sea water would give a faster quench). Although I'm not sure what the boy’s hair colour has to do with it (but it worked so best not to question it).
This need for a medieval smith to have years of practical experience is of course a function of lacking knowledge of the science involved, but it is also because there was little way of assessing temperature and material quality, other than than by eye.
I was taught basic blacksmithing at school (I doubt in the modern risk averse culture that 14 year old boys are allowed to hammer red-hot bars of metal to make pocket knives for themselves … but in the 1970s I was). Having studied metallurgy at university I now have a pretty good idea of the science involved, but at 14 I was essentially taught the practical side only, but that was still sufficient to make things. For instance to get the correct degree of temper for say a knife blade as opposed to a screw driver blade, one heated it until the surface oxide was "light straw yellow", for the knife blade, rather than "dark blue" for the screwdriver. (Don’t quote me on those colours I’m years out of practice). Similarly having worked in the metallurgical industry, although of course I would always rely on accurate temperature measurement devices (thermocouples, pyrometers etc) I could roughly estimate the temperature of things in a furnace by the colour of the light. My father, who was a lecturer in engineering but had once been trained in blacksmithing, was even able to tell the carbon content of a steel by the shape and colour of the sparks it produced when touched to a grind stone. But I digress.
Nevertheless going back to the artisans working at Guédelon castle, this need for years of practical experience to be able to judge correctly by eye, largely explains why an armourer, a farrier, a black-smith, a copper-smith, a bronze-smith, a plumber (ie a plombier or lead-smith) etc… were all distinct trades, despite them all working with metal.
It's fascinating to see all these old techniques in use in those Guédelon films.