How are part-to-part ratios different from part-to-whole ratios?

Part-to-Part ratios give the relationship between 2 distinct things/groups. Such as the ratio of male to female in a class, or ratio of water to sugar in a drink etc.

Part-to-Whole ratios give the relationship between one group/thing to the whole population. Such as the ratio of male to total students in a class, or the ratio of cups of sugar to total cups of drinks etc.

Note: If there are 2 parts in a population, we can essentially write 3 ratios. For example, the ratio of male to female in a class is 4 to 5. So in total there are 4+5 = 9 parts (whole). So we can say:

ratio of male to total population is 4 to 9

ratio of female to total population is 5 to 9

Also,

ratio of male to female (as stated before) is 4 to 5

3 ratios.