Nicotine shots are meant to be used with a shortfill (a nicotine free bottle of e-juice). Nicotine free e-liquids arn't regulated under the TPD (European Union Tobacco Products Directive). This means that the maximum volume of 10ml per bottle doesn't apply to shortfills, and they can be any volume. A shortfill bottle always leaves space for at least one nicotine shot to be added, for example a 60ml shortfill bottle contains 50ml of e-liquid, and a 120ml shortfill bottle contains 100ml of e-liquid.
A nicotine shot is either 18 or 20mg of nicotine per millilitre. When adding the recommended amount you will usually end up with a final e-juice of 3mg nicotine per ml. Some shortfills are made to be mixed to 6 or 12mg. Nicotine shots are regulated under the TPD which means they can be at most 10ml and 20mg/ml of nicotine. The nicotine can be either freebase (most common) or nicotine salt, based in either PG, VG or a mix (VPG, usually 50%).