There are official courses run by most professional bodies (in Australia, the KBDA and HIA to name 2) or even Interior Design degrees but the best designers seem to pick it up by themselves.
In my case, I’m primarily IT having been a computer guy from the late 70s but I renovated a few UK properties designing the kitchens myself and installing over-the-counter flatpacks (inlcuding the plumbing and electrical) but when I migrated to Aus, I needed to invest/run a business for 2 years so I bought a Kitchen Franchise and ran that for 7 years designing probably over 2000 kitchens/laundries and selling over 700 of them. I then sold the franchise back to the parent company and now work for them supporting 2020 Design and IT in general.
Mostly it’s about
Clearances
Work Optimisation
Appearance
Generally I design with as many drawers as possible and avoid complexity and only get ‘fancy’ when the client wants Ultra Modern or Traditional (called Federation Style in Aus).
Clearances are everything – see attached pic (example off the web – not one of mine!)

