One of the popular, and free, places to learn about Dynamics 365 CE is the Dynamics 365 CE SDK, which is available online. We can access all the documentation for Dynamics 365 CE from https://docs.microsoft.com/en-in/dynamics365/, like so:

This documentation provides all the details about Dynamics 365 CE. As shown in the preceding screenshot, we can see different navigation options to get details about Dynamics 365 CE. We can see what has recently been released in the What's new section; we can see best practices for Dynamics 365 CE development under FastTrack your deployment; and we can find eLearning courses from Microsoft on this page.