I'm in the process of building the D-Bot core XY printer, and I was hoping to know if the Z-axis 'zero' is near the hot end or near the bottom of the printer furthest away from the hotend? In this printer the Z-platform moves up and down and the nozzle stays at the same height.
Generally, Z-axis zero is when the hot end and the build plate are close to each other. Some printers keep the hot end up at the top, and bring the build plate up to reach it. Other printers keep the build plate at the bottom, and lower the hot end down to reach it. As such, "Z axis zero" doesn't specify top or bottom of the physical printer, it just specifies that the hot end is right up against the build plate, ready to print its first layer. In the printer you describe, where the hot end remains at the top, that's where Z zero is.
In terms of the model being printed, Z axis zero is always the bottom of the model, the first layer to be deposited on the build plate. The numbers increase from there, either lifting the hot end up, or pushing the build plate down, depending on the design of the printer.
Yes, Z-Zero is typically at the "top" of the printer, closest to the nozzle(s). X and Y zeros are also typically in the lower-left corner of the buildplate.
However, the XY zeros are re-interpreted in slicing software to produce cleaner G-Code as it's sometimes difficult to read G-Code in negative coordinates. For this functionality, slicing engines utilize the machine build space length and width.
So if you encounter a situation where your machine "over travels" in either -X or -Y direction during startup, verify that the length and width of your build space is correct in your slicing engine.