Formed into the wall before any concrete moves, and permanent once it does
A brick ledge is not something added after the wall is poured. It gets built into the form itself, an insert set at the height and depth the veneer calls for, before the pour ever starts. Once concrete fills the form, that shape is permanent. Get the ledge width or the step height wrong and the mason is the one who finds out, standing on site with veneer that does not sit right.
A wall poured without a ledge it was supposed to have is not a five minute correction after the fact. Chiseling a shelf into cured concrete is possible but rough, and it never looks as clean as one formed in from the start. That is the reason the veneer plan needs to be in hand before we form the wall, not decided once it is already standing.
How deep it steps down, and why that number is not standard
The step down is set by the veneer thickness plus the air space and flashing the mason's detail calls for, not by one depth we use on every job. Thin stone veneer wants a different ledge than full brick does, and a heavier stone veneer can want a wider ledge than either. That measurement comes off the plans or the mason's detail, and we build to it rather than guessing at a typical depth from the last job.
Where it shows up on a real pour
On a foundation wall, the ledge runs the length of whatever elevation is getting veneer, formed in at the same time as the window and door openings and the anchor bolt layout, since all of that hardware gets set into the same open forms before concrete moves. On a slab edge, it is the same idea at a smaller scale, a stepped edge instead of a straight one. Either way it gets checked for level and for the right step height before the pour, because a ledge that is out of level telegraphs straight into a wavy first course of brick, and there is no fixing that once the mason is stacking on top of it.