Are the ingredients listed under "Nutrition Facts" on products per container, or per serving?

Example: Sugar -- 20 grams
Fat -- 15%

It is per serving. It clearly says that at the top of the label.

Typically it is measured by serving not container.

Also, important to look at how much a serving size is. Often, people eat or drink more than what an actual serving size is.

Also realize that there isn't a set guide to what is a servering. A lot of packaging put what they think is reasonable and not what is actually consumed. Take certain bottles of pop or bags of chips.

There is a lot to take into consideration when looking at those.

It can be per serving or per 100 grams and it should be clearly noted on the label.

Honestly though, who's ever had a single serving of chips? Most people will just demolish the entire bag. Half of it is marketing. For example, a light product has to have less than .5g of oil per serving or something ridiculious like that, but if your serving size is 0.5g on a cooking oil than any filler you add will reduce it so you can have a light cooking oil even though you're consuming essentially the same stuff.

It is one thing to read it, but to understand it and know what it really means might take a while longer.