If you want to learn using the online tutorials, you’re in the wrong place. Instead, please go to the Developing Secure Software (LFD121) entry page so you can learn how to develop secure software. It’s free, and its certificate of completion is free.
The OpenSSF page about this course provides some broader context about the course. The learning material also available via edX. but the edX certificate of completion is not free.
However, if you want to view the course material as a book, you’re in the right place! Please view the Developing Secure Software (LFD121) course material as a book.