For this one, it’s more wind shear and ice cleavage/hot expansion that does most of the erosion from what I understand. Everything in the area is mostly sandstone or similar so it erodes relatively easy. Some layers are ‘softer’ than others so in the case of this one the lower area eroded easier than the upper boulder, leading to what you have now. At one point it would have been much larger and over time pieces have broken off to where you are just left with this.

Slot canyons are formed by water flow closer to what you’re talking about though, and could also lead to a similar outcome as this, but it would likely be surrounded more closely by other rock walls, as well as probably being smoother.

For arches it could go either way; a small cavity develops which just keeps eroding until you are left with two large bases connected by a bridge. You could also have water cutting its way down to form a slit canyon but there was a tougher layer toward the top that didn’t fully erode before the flow got too low to hit it and kept carving beneath.