It's rather a Fishy tale.... In fish oils the two-key omega-3 fatty acids are Docosahexaenoic acid (DHA) and Eicosapentaenoic acid (EPA). Some plants are rich in another type of omega-3 fatty acid, Alpha-Linolenic acid, which the body is able to convert to DHA and EPA.
There's a huge market in Fish Oil mainly due to advertising. Estimated sales for 2022 is around the $4 Billion mark.
The scientific evidence is mixed. Research has shown that omega-3 fatty acid supplements did nothing to reduce heart attacks, strokes, or deaths from heart disease in middle-age men and women without any known risk factors for heart disease. Note that eating fish and taking fish oil capsules are two different things.
PS: The differnce in Fish oil & Cod-Liver oil is how these are made. Fish oil is made from the body tissue of fish, whereas Cod Liver oil as the name imples is extracted from the Livers of the Cod Fish. The two contains different percentages of EPA & DHA.