Nic shot is a term given in common language to a small 10ml bottle that contains 18 milligrams of flavourless nicotine strength solution (PG/VG). This solution is then added to a short fill or some sort of an e-liquid that is nicotine free. It then creates the preferred strength of nicotine for the vaper. Your short fill bottle will already have extra space for your nic shot other than the e-liquid. You just got to pour it in.
Nic salts on the other hand are directly obtained from the crystals found naturally in the tobacco leaves. These are considered significantly easier to vape; however, care is required when using them as these are relatively less swiftly absorbed into the system. Nicotine shots can be the key to a quick and a great vaping experience when paired with the flavour of your choice. However, in all cases do monitor the amount of nicotine you intake.