range, reach, extend, cover
亙 centers on spanning across a space or scope: ranging over an area, reaching from one point to another, extending coverage, or covering a subject.
亙 is an ancient character depicting a boat or vessel spanning between two shores, representing the idea of crossing or extending across. The modern form retains the top and bottom horizontal lines as the two shores with the central shape as the connecting vessel.
The top and bottom lines look like two distant shores, and the middle shape is a boat or bridge reaching across. 亙 means to range, extend, or cover the space between.
For コウ, imagine a long covered corridor (こうろう, 廊下) extending between buildings: こう -> コウ, and the corridor covers the distance.
from long ago up until the present day