If Liquid A and Liquid B don’t mix, what would you guess about the liquids
both have different polarity. Polar substances can dissolve/mix with each other. non polar substances can dissolve/mix with each other. you cannot mix both polar and non polar substances.
example: Oil is non polar and water is polar. you will find two distinct layers because of the different polarity.
search on polarity of substances. miscible and immiscible can also help.
hope that helps