In relation to the monster ability "web walker", in this case from a giant spider:

Web Walker. The spider ignores movement restrictions caused by webbing.

The only other mentions I can find of webbing come from the web spell:

Web 2nd-level conjuration
Casting time: 1 Action
Range: 60 feet
Components: V, S, M (a bit of spiderweb)
Duration: Concentration, up to 1 hour
You conjure a mass of thick, sticky webbing at a point of your choice within range. The webs fill a 20-foot cube from that point for the duration. The webs are difficult terrain and lightly obscure their area.
If the webs aren’t anchored between two solid masses (such as walls or trees) or layered across a floor, wall, or ceiling, the conjured web collapses on itself, and the spell ends at the start of your next turn. Webs layered over a flat surface have a depth of 5 feet.
Each creature that starts its turn in the webs or that enters them during its turn must make a Dexterity saving throw. On a failed save, the creature is restrained as long as it remains in the webs or until it breaks free.
A creature restrained by the webs can use its actions to make a Strength check against your spell save DC. If it succeeds, it is no longer restrained.
The webs are flammable. Any 5-foot cube of webs exposed to fire burns away in 1 round, dealing 2d4 fire damage to any creature that starts its turn in the fire.

Does web as a terrain feature need to be treated as if it was cast by the spell?

For example, if I want the spiders to have covered the floor with a thin (that is, not the absurd five feet deep as specified in the spell description) layer of web, only enough to obscure any pits etc, could that just be counted as difficult terrain? My intention would be to have the webs as a difficult terrain feature that lightly obscure the walls and floor, but not requiring saving throws to avoid being restrained as per the spell. The spider's web sense ability would still function as normal, and players could easily remove the web with fire and the like.

In relation to the monster ability web walker, in this case from a giant spider:

Web Walker. The spider ignores movement restrictions caused by webbing.

The only other mentions I can find of webbing is in the web spell:

Each creature that starts its turn in the webs or that enters them during its turn must make a Dexterity saving throw. On a failed save, the creature is restrained as long as it remains in the webs or until it breaks free.

A creature restrained by the webs can use its actions to make a Strength check against your spell save DC. If it succeeds, it is no longer restrained.

Do webs as a terrain feature need to be treated as if it was cast by the spell?

For example, if I want the spiders to have covered the floor with a thin (that is, not the absurd five feet deep as specified in the spell description) layer of web, only enough to obscure any pits etc, could that just be counted as difficult terrain? My intention would be to have the webs as a difficult terrain feature that lightly obscure the walls and floor, but not requiring saving throws to avoid being restrained as per the spell. The spider's web sense ability would still function as normal, and players could easily remove the web with fire and the like.