Learn French: La plupart des employés sont très aimables. - Most of the employees are very friendly.

Even though La plupart (most/the majority) is a singular phrase, the verb agrees with the noun that follows it. Since employés is plural, the verb takes the plural form sont. You can think of it as focusing on the employees themselves rather than the concept of "the majority."

I learned that quantity words usually just take de (like beaucoup de). Why is it des employés here?

You are absolutely right that most expressions of quantity drop the article (like beaucoup d'employés). However, la plupart is a major exception! It always keeps the definite article (le, la, les). Therefore, de + les employés contracts into des employés.

Why does aimables have an -s at the end?

In French, adjectives must agree with the noun they describe. Because employés is plural, we add an -s to aimable. Also, since aimable already ends in an -e in its base form, we don't need to add another one for feminine groups, just the -s for the plural.

What is the difference between aimables and sympathiques in this context?