This is the same dissonance as the post about "never write bugs". The premise is wrong to think of code as something you can fit in your brain like you remember the shape of a new building you frequent or a persons face.

You need to both remember the processing done in the code and the structure. The structure will need your working memory in the language center as its n-dimensional and not representable with a closed surface 3D model - at maximum you can do graphs which well dont look like much you will be able to draw from memory.

Remembering state and data needs your whole brain to debug what will happen when you run the code.

All people here telling you they can train this train either an abstract concept like functional programming or work with software in their field which will have a similar scope - you will NOT be able to easily comprehend code written in another style or purpose. Don't be mistaken doing 10 years of mobile apps, ERP or whatever would allow you to follow any C-written code for systems or sth.

Fitting code as an abstraction in your mind is literally neurons growing new pathways - its expensive, no one likes doing it and if you are not in the same field you will try to bend it into your domain or not engage much.

Trying to find general assumptions here will not work - if you could condense n-dim rulesets to some general principles - you would simply do so and refactor the code.

Thinking theres no way to debug abstracted, proveable code you can not fit fully in your working memory is wrong, because you only need to test each function / morphism to be correct, not the whole thing. Its certainly doable to write systems that interconnect 30-40 entities you need NOT remember all at once and still end up with bugfree code because each part is constrained to strict interfaces and types.

That's how rulesets with bigger scopes work - no one designs the aircraft carrier all alone - there's restrictions and interfaces everywhere, not a single engineer knows the whole thing down to the molecules and electrons. Still it works.