Extensions are developed and distributed by multiple authors. Some extensions are free and open-source, while others are only available commercially.
Use the Extensions Overview page to browse through existing extensions (both free/open-source and commercial) using tags and filters.
Most open-source extensions are hosted on GitHub. Once you know which extension you would like to use, follow the link to its GitHub repository, download the code to your computer, and unzip the package.
Open-source extensions can be installed and updated using Mechanic (an extension to install other extensions). See Installing extensions with Mechanic.
In addition to the free and open-source extensions available from GitHub/Mechanic, there are also several commercial extensions available from the Extension Store.
If none of the existing extensions suits your needs, you can develop your own using Python. Or hire someone to do it for you.