The One-Line Resistor
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1. The way ordinary through-hole film resistors are made is to deposit a carbon or metal film on the substrate, then trim it in a spiral pattern to set the resistance. To make a zero-ohm resistor, then, all they do is... not trim the film. Skipping a step is very easy for them to do!
2. The components must have identical body shapes to ordinary through-hole resistors, or the automated placement equipment can't handle them. It is possible to use wire jumpers as well, but that's a different machine. (Which absolutely exists and is used, it's just only found in assembly shops big enough to buy them.)