When thinking of the experiences or backgrounds needed for any specific project, don't let assumptions trip you up. These terms can help with your communications.
Configuration – knowing of and Answering all the questions related to setting up the system for a specific customer. These include all the options under SETTINGs including but not limited to items such as turning on auditing, configuring number formats, turning on relevance search, setting up duplicate detection, configuring territories, setting fiscal year end and more. This is done on every Dynamics 365 project (hopefully) and there are more than 50 questions.
Customization – A functional consultants power to configure the system for specific business models such as renaming entities, adding entities, adding system charts, lists/views, adding workflows, creating business rules, formatting the data entry fields and forms, creating PowerApps, setting up relationships between entities and more. The term customization often waffles between configuration and extensions depending on who is using the term.
Extensions – extending the functionality of the system using developer resources and many, many, many different code options (.Net, C++, JavaScript, AngularJS, and about 30 more) and includes items such as creating an integration layer with queuing to manage external data integration (although this is shifting to configuration), adding new advanced features, adding advanced automation, extracting/manipulating and resaving data from numerous entities, onSave/onChange/onLoad have the data jump through hoops and blink, etc. The platform is built to be extended and extensions upgrade as they are built using a predefined best practices Software developer kit (SDK) from Microsoft.
The world of technology is deep and wonderful.