When you break up with someone do you explain why or do you just make a break and let them figure it out?

Depends.
If it’s a sociopath or a narcissist, you just go away.
If it’s a normal person, you can tell them why if you think it will help YOU. Or if you want them to learn from their behavioral errors. Or you want to maintain a relationship for some reason, like kids or other social connections.

It is best to give them a reason..how would you want someone to break up with you

You owe it to the other person to give them closure.

tell then whatever you like

Lol i used to break up until they figured it out but I quit doing that, now I have an adult conversation