Two kinds of diesels out there: Direct injected (Detroits, Cats, most Cummins, most anything bigger) and pre-combustion chamber aka indirect injection. Most (or lots) of small engines use pre-chambers with glow plugs. Direct has no glow plugs, but some have inlet air preheaters in air passages leading to cylinders (Cummins B and C, some).
Direct shoot the fuel right into the cylinder. Pre-chamber shoot the fuel into a chamber off to the side of the cylinder, with a glow plug stuck in it. Pre chamber diesels most all need some glow to get a good start. Direct needs no glow plug, but even those below a certain temp it simply will not start without help.
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