Extensions are code blocks that can be used to increase the functionality of a standard e-commerce application that is open-source or open-market. Standard e-commerce platforms are designed and developed keeping in mind the larger market and common trade scenarios.
For industries that have their own personalized needs, they can either buy an extension that fits all their requirements or get an extension developed for personalized use. These extensions are usually built for a specific function, task or industry and are developed based on the market understanding or needs.
While most e-commerce platforms have their own marketplace where store owners can find their desired extensions, there are also service providers that can develop personalized extensions that are integrated into your e-commerce store.