What I've learned from vintage tailoring magazines is that in the olden days cutters, after working as a coat/ trouser maker for a while, would go to a cutting school (Tailor and Cutter in the UK, Müller & Sohn in Germany, probably similar in other European countries) to learn pattern drafting and fitting skills.
So in theory they more or less had the same basic knowledge after completing the course. Since bespoke tailoring is in decline those schools have disappeared, at least in the UK, although from personal experience I'd say that what is left of Müller & Sohn is not even a shadow of its former glory.
This is a map from 1938 and those black marks do not represent a single tailor, but a tailors guild, so anything from 5 to hundreds of members:
Needless to say that you'd struggle to find a fraction of those guilds today.
German bespoke tailors association had a group of top bespoke tailors to decide on the silhouette for the upcoming season (usually just some smaller tweaks of the existing lines) based on actual fashion trends.
Not to mention constant competitions on local and national level (quite common in other countries, too).
This lasted well into the 70's, but with the rise of RTW and MTM (and the older tailors dieing) it eventually went the way of the Dodo.
Tailors were split into 2 general groups: City tailors and the rest. While the first group would be the one following the fashion trends and more expensive overall, the rural tailors would make for less demanding customers, often at a much lower price point and with some shortcuts in the make.
A few tailoring houses in larger cities had a large number of tailors in-house, while many others were much smaller, often just a one-man band or perhaps with one or two apprentices at different stages.
The better tailoring houses usually attracted better tailors, often from other countries like Italy, Greece and even Hungary, so there was definitely an exchange of skill and knowledge in those days and overall standards were pretty high, even among rural tailors.
Today there are only very few, mostly small bespoke tailor firms left in Germany. Volkmar Arnulf in Berlin is probably the last of the big names along with Dietl in Munich.
This is one of the more fashion oriented bespoke outfits from the 70's:
Or for the fans of the more classical version: