Howdy, Ayan!
Well, the value of UML diagrams is that they communicate design choices, or even situations. This means that
you should create only the diagrams necessary to communicate things to other people. If you're going to create a sequence diagram, but you are not going to discuss it with nobody, then it isn't necessary.
You can create as many class diagrams as necessary, each diagram with a group of classes that make sense to take part on the same diagram. All layers of your application can have one or more class diagrams.
To represent classes of other libraries or frameworks, what I usually do is, I add a stereotype, identifying the library that owns a class. For example, if I wanted to represent the ApplicationContext interface, I would add an interface named ApplicationContext with the stereotype <<org.springframework>>.
One thing that I do all the time at work is create class diagrams on a whiteboard with some classes (sometimes 5 classes, sometimes with 20 classes, it depends) and discuss what will be done with the other members of my team. Thus, we use it as a tool to discuss design.