The formula for density is: ρ=mV
=> where ρ is the density in either kgL or the variant conversions - gmL, etc, mass is the mass of the object in kg or other variants, and V is the volume in L or other variants.
Mass of an object can be measured in either g or kg by using a scale.
Volume of the object can either be calculated by V=lwh or through a water displacement experiment - getting a beaker of water and placing the object in the beaker. Subtract the new measure of water with the original measure. The difference is the volume of the object.
In the video, the density was found online, and the volume was found by subtracting the final volume of water (425mL) by the initial volume (150mL), equaling 275mL - the volume of the object.
When calculating the mass (in the video), the mass will come out in grams because of the relationship between mass and volume. You can convert the variable to another, like mL→L, but then your mass will be in kg.
Hope this helps :)
