Where do the class methods come from?

In a class diagram, how do I know what class methods to add? I’m not sure what the connection is between class methods and other (i.e. Activity and User Case) diagrams. My guess is that they come from Sequence Diagrams, but then I’m not sure how sequence diagrams relate to the Activity and User Case diagrams. I’m trying to understand how all the design pieces fit together. How do diagrams connect or flow into each other? I’m presuming they’re related.