Somewhat different meaning. Defer means you are wanting someone else to do something for you, or make a decision for you, because maybe they have more experience than you. For example, “I don’t know what I will say in this meeting so I’ll defer to you.” Or, “I can’t make a decision. I’ll defer to you. You know what you’re doing.” It can also mean to delay, or postpone something, like “I cant do this project right now so I deferred it to next week.” But “leave it up to” means passing a responsibility to someone else for whatever reason. So you could say, “I don’t know what I’ll say in this meeting so I’ll leave it up to you.” In this case, it means something similar to defer, but defer means the other person is more of an expert. I use “leave it up to” for informal situations. Like, “I’m going to leave the dishes up to him.” He’s not an expert at washing dishes necessarily (could be) but I just mean that he’s going to wash them, not me. The sentence, “I am going to leave it up to in many things” is grammatically incorrect. Instead, you would say, “I am going to leave it up to you” or “I’m going to leave these things up to you.” Of course, you can change out “it” or “these things” to whatever noun or pronoun you want. And you can change out “you” for whatever noun or pronoun you want. For example, “I’ll leave it up to her to clean up her toys.” I would never say, “I’m going to defer to her to clean up her toys.” Too formal and it implies that she has “more experience” in cleaning up toys. Hope this helps! Others want to chime in?