How Can I FInd the Usages of Different Jupyter Extension Developing APIs

I am now a new developer of jupyter notebook extensions. When I look at the codes of other extensions, I was really confused about the all kinds of APIs like "Jupyter.keyboard_manager.command_shortcuts.add_shortcuts", "cell.element.find()", etc. I tried to search the docs about these APIs but find nothing. If I don's understand the usage of this, it's hard for me to create a new extension. So I am just wondering how can I know the usages?

Start by reviewing the extension API examples named in the issue, including Jupyter.keyboard_manager.command_shortcuts.add_shortcuts and cell.element.find(). Identify where these APIs are currently documented or defined, then document their usage and examples so extension developers can find the relevant guidance.

Written by the indexing model from the issue text.