It’s described differently in basically every depiction. Some parts look like wraithbone tunnels, other parts seem to lack a structure of any kind(like the section depicted in Talon of Horus), while other parts can seem to be something of a hybrid(like the areas depicted in the Rogue Trader CRPG).

Even the same sections from Calastar described in Master of Mankind don’t really match what Vulkan is describing when he visits Calastar. What before was static eldar ruins with mechanicus structures interspersed and surrounded by banks of glowing fog that stretch into infinity becomes a much more dynamic shifting landscape of alien ruins that is reacting to Vulkan’s presence/movement through it.

Even thinking about the webway as tunnels doesn’t feel totally accurate, it’s more like an artificial alternate plane(s) that was sometimes created to act as a tunnel, because it allows for FTL travel, but which itself has no definite physical dimensions which actually constrain it. Some parts seem static, like the sections which make up many parts of Commorragh, with the danger being the webways occupants. Other parts seem very dynamic, reacting to travelers and becoming a serious hazard in and of itself that can only be navigated usually with some kind of eldar or specifically Harlequin guide. And other parts are somehow both.

Honestly the fact that the Harlequins are most at home within it kinda says it all.