"The well-to-to" = wealthy people/rich people. "The beautiful" may mean for women, but in this form can often be used for both genders. It often means "lovely, young people with too much money and time." For example, there is a famous American book called "The Beautiful and the Damned." "The difficult" is not commonly said. We would add the noun with this adjective -- "the difficult child" or "the difficult man." In English it is common with some adjectives to end with the adjective and to imply the noun without saying it. What the noun is depends on context, but if you cannot find the context, "people" is usually the noun we are meaning. We also use the word "ones." We don't do this commonly with all adjectives. Most often, we use this construction with adjectives that show a group identity, like social or economic status. So, "difficult" would not be used this way and we would add the noun.

Jamie's answer is great but has a typo. It is "well to do".

I hope my other explanation helps.