Moodle academy publishes a year-end summary, which includes all freely available developer courses:
https://moodle.academy/course/index.php?categoryid=4

1. Moodle Developer Basics program and certificate
2. Moodle Developer Skills program and certificate
3. Set up your Moodle Development Environment
4. Web Output Essentials
5. Moodle’s Modular Architecture and APIs
6. Moodle Access and Security Essentials
7. Accessible Development Practices
8. Unit Testing in Moodle
9. Update Plugins to Moodle 4.0
10. Moodle Architecture
11. Moodle Database API
12. Moodle Page Layout and Site Navigation
13. Access Control and Security in Moodle
14. JavaScript (ESM) for Moodle
15. Make Your Plugin Moodle App Compatible
16. Creating TinyMCE Plugins
17. MoodleNet for Developers​