Dwarves can only build walls in places where they can stand at a location perpendicular to the wall segment while standing at the same level as the new wall segment. When these conditions are fulfilled they can be seen actually standing diagonally from the new wall segment or on a ramp leading up to that level, but they can not build those walls if the could not have stood at the legal locations. Thus, you can see dorfs stand on a ramp and build wall segment up to the left and up to the right, but they'll fail to build the one up ahead (where they could have stood to build the ones up to the left and right respectively). Thus, you typically need stairs to build at higher levels (and stairs work more like elevators than ladders in that you need a "platform" on the level you want to reach: an up stair does nothing in itself as you need a down (or up/down) stair at the level above it [which CAN be build from below] to reach that level.
Also, if dorfs have to walk on the wall at the level below to reach the building site, you have to micro manage the building so the wall segments are built in the correct order, or you'll get holes where designated wall segment positions cannot be reached because other wall segments are build in locations blocking the access to the building site. You can get around the micro management by building long single tile wide bridges as scaffolding, but at the cost of extra building work.
I don't do videos, so I don't know if this answers your question.