magento2 – how to find class or interface which plugin will work on

hie, i am very very new to Magento development.
I am trying to make a plugin for a check out process. One of the steps is to create a di.xml file which indicates the class or interface to be observed. I am lost as to where i can actualy find this information (observed type). Where is the class of interface that does the checkout(example), how do i find it?. Do l search through all the modules in the magento vendor folder until i find something?

type name=”{ObservedType}”>
plugin name=”{pluginName}” type=”{PluginClassName}” sortOrder=”1″ disabled=”false” />

your assistance would be greatly appreciated