well there's a subtle difference:
"figure out" is used when you are using pieces of information and you pull them together like a puzzle
i.e. "the detective figured out who stole the diamond"
understand is being able to follow a line of thinking, mostly because you pay attention to new info and you can connect it quickly with previous
i.e "you can understand math if you start with basic operations"
vincentJuly 2019great answer, thanks it's subtle but important to know