These operate in the entire Chrome Browser
Optional:
These are like extensions, but for specific file types.
A Google Doc Add-On must be installed to a specific Google Doc. Then it will be accessible from any of your Google Docs. Here's a good example:
There are many more. Find them by going to the Add-Ons menu in a Google Doc.
A Google Sheet Add-On must be installed to a specific Google Sheets file. Find them by going to the Add-Ons menu in a Google Sheet.
A Google Slides Add-On must be installed to a specific Google Slides file. Find them by going to the Add-Ons menu in a Google Slides file. Here's a good example:
A Google Forms Add-On must be installed to a specific Google Form. Find them by going to the Add-Ons menu in a Google Form. Here are some good examples: