I recommend checking the skills measured for this IoT Developer exam.
From there, start with the Microsoft Learn IoT learning paths and additional modules.
At least this will give you a head start regarding the exam.
Next to that, read the documentation of the individual topics for more in-depth background information and examples. See also my blog with my personal study guide.
Note: Dynamics 365 is currently not part of the exam outline.
(please don't forget to mark helpful replies as answer)
Best regards, Sander