I have a redhat subscription and I am currently studying for the EX200 exam. One of the objectives is "Modify the system bootloader".
In the redhat material, what topic is it covered under?
If you already have the learning subscription (and have time). I could recommend you to go through the RH134 (System administration II) "Chapter 8: Control the Boot Process" . EX200 relevant material is RH199 (accelerated version) and you find this in "Chapter 9: Controlling Services and the Boot Process".
If time is not a problem, I highly recommend you to go through the RH124 and RH134, paying attention to the written and video material (because they complement each other) and to the references at the end of each subject, such as guides, commands manual and so on.
After that, as a review, just focus on the labs of each chapter and the comprehensive review. Like Ricardo da Costa says in the videos, "it is important to get the hands dirty on the keyboard" and "work smarter, not harder".