How to ensure fat stays in meat/fish? I'm looking for a cooking method which retains fat and oil.  When I grill, boil, simmer etc I notice most of the oil leaves the meat. 

Which method is best to ensure fats and oils are retained during cooking?

I heard that if you marinade in olive oil before grilling that will prevent fat from coming out.  Is it true?

Can I grill or bake at lower temperatures to get desired result?  will the meat still be cooked to the safe temperature if done this way?
Fat will melt at high temperature (temperature depends on the type of fat); there is not real way to get away from that.

If you pan fry a steak, the fat will melt into the pan; that is why most people suggest a good "marbling" of fat in the meat so not all of it will melt away.

One cooking method, that I think will help keep most of the fat in the meat is low temperature vacuum cooking. 

Put the meat in vacuum sealed bag and cook at low temperature (check instructions) in a water bath; after a while, the meat will be cooked and the fat will not have melt away; just finish in a pan to sear the outside.

See:
https://www.thekitchn.com/sous-vide-cooking-this-is-the-best-steak-ive-ever-had-197056