"Hold it" generally has an authoritative or confrontational usage, like if you were confronted by a police officer or by a parent when you're doing something bad. It's an imperative instruction or an order. "Hold it! Get on the ground." Or "Hold it, put that cookie down. "Here, hold it," would just be a simple instruction, like if you handed a coworker a tool. "Hold on" is a casual expression. Usually telling the person to wait. "Hold on, don't leave without me, I'll be there in 5 minutes." "Hold on, let's make sure we have the keys before we leave." "Hold on, when we talked last, I thought we were going at seven pm." A more colloquial version of "Hold on" is "Hold up," same meaning, but used more by younger people, but it's common enough that it's not slang.

If you are asked to "hold it" the usual meaning is for you to temporarily stop whatever you are doing until a problem has been resolved. It is usually expressed as a command ("hold it!"). If you are asked to "hold on" it could also mean to temporarily stop doing whatever you are doing (with a bit less urgency) or it could represent a request for you to wait for a short time (until a fuller response can be given) or to endure some level of difficulty (to not give up)