Schemes and tropes are both rhetorical devices, but they have different functions.
While schemes are related to word order, syntax, letters, and sounds, tropes are related to the meaning of words.
Onomatopoeia is an example of a scheme that deals with sounds. Words like “boom” and “howl” are schemes in which the sound of a word emulates the sound of the thing that the word describes.
Oxymoron is a trope that consists of a self-contradictory combination of words, such as “friendly fight” or “falsely true.”