> Since when did “scratch-built” become equivalent to “built from scratch”? As a non-native English speaker, do I need some more English classes?

As a native English Speaker, "scratch-built" is definitely not a common term (I was confused when I read this).

It's somewhat common in the context of cooking. A recipe may refer to something as "scratch made" when its "made from scratch".

In this case, its an extension of the phrase but using the word built instead of made.

Ref: https://grammarphobia.com/blog/2017/02/scratch-made.html

They are both equivalent. Scratch-built is perfectly cromulant.

My father used to talk about 'Scratch-building' his WWII dioramas and model vehicles/tanks. While you could use a phrase like 'Scratch-cooked' rather than cooking from scratch, I've never heard but someone would not think the former meant something different to the latter.

I've heard "scratch-made" applied to cooking before.

I see what you did there. (cromulant was coined in a Simpsons episode)

As a native English speaker I can tell you it's not normal phrasing. You're not taking crazy pills.