Although some of the resources need updates due to latest Magento versions, most of the stuff still applies, check this out:

https://github.com/colinmurphy/magento-exam-notes
<br/>
https://github.com/magento-notes/magento2-exam-notes (this one has more recent updates)

In the readme of the repos above you will find references to other resources as well.


Wish you best of luck!