ELI5: When cardiologists say to take fish oil, or eat salmon, is it to replace a nutrient, or to repair damage that causes cholesterol to rise. Like, what is the goal of the salmon here?

If I were to tell you I had bad cholesterol and heart symptoms (tachycardia, arrhythmia, benign stuff), with occassionaly hypertension, but mostly normal to low BP, would fish oil bring down the cholesterol, or is it just replacing a poorer choice of protien? Asking because I am rapidly losing weight and it occured to me that they want me to eat like I”m obese. But I’m 112.9 [ounds at 5’10” (55f). Kinda concerned. In September I weighed my highest at 149. I lost another pound since Tuesday. I’m scared. Doctors are not. ?!?