I’m not sure I understand this statement. In both humans and mammals, the term fat is really just a convenient way of saying triglyceride or, nowadays, triacylglycerol.
Fatty acids refer to one component of the triglyceride molecule - there are three fatty acids plus one glycerol in every triglyceride molecule. The particular fatty acids used to join with the glycerol vary. Depending on your diet, the fatty acid can be saturated (eg. palmitic acid), monounsaturated (eg. oleic acid), or polyunsaturated (eg. linoleic acid).
BTW, fatty acids are also what are used to join to cholesterol to get the latter into cells. Depending on the type of fatty acid used, eg. saturated vs. unsaturated, there are major differences in cholesterol metabolism.
Very efficient! Evolution is on our side. In fact, by making it hard for sugar to get into muscles (insulin resistance and/or “thrifty genotype”) there is a profound secondary effect that causes even more fat to get into fat cells.
Fat in fat cells is simply triglyceride. Not in solution AFAIK. It got delivered to the fat cells on particles called lipoproteins, and gets into the fat cells via the action of insulin.
Wanna know more?