How to understand "ideographic" decomposition of characters like 不 and 与
my best guess:


  U+4E0D    不   ⿱一③


不 is a top-bottom structure ⿱, with top component "一", ③ means components with three strokes, not yet defined.

you may look at these two:


  U+4E1A    业   ⿱④一


业 is a top-bottom structure, with the top component with 4 strokes, then the bottom component "一"


  U+4E1D    丝   ⿱⿰②②一


丝 is a top-bottom, and further, the top component could be divided into left-right component. so, ②② means two components, each with two strokes, left-right relationship, on top, then bottom component is "一"

have fun :)