by measuring the object
Solids have definite shapes and definite volume.
Yes many different shapes can have the same volume
There is no formula for measuring the volume of irregular shapes. The mass can be measured by using a weighing scale.
water
There are none because volume only relates to 3 dimensional shapes and not 2 dimensional shapes as in the question.
The volume of anything is always the area of the base of the object times the height. Therefore, to find the volume the object must have a height, which makes volume work with all 3D shapes. If one is looking at a 2D shape, then there is no height so a volume cannot be calculated (note: areas may be found of 2D shapes).
Solids have definite shapes and definite volume.
0
You do not. As two-dimensional shapes geometric squares have area and no volume whatsoever.
Put the object in a bath of water and see what volume of water is displaced.
No volume is for 3 dimensional shapes and area for two dimensions.
There are formulas for regular shapes. Not all shapes have formulas; for these you can fill them with liquid and measure or weigh the contents.