The most important documents for understanding the exam structure and requirements are provided by (Federal Institute for Pedagogical Measurements):
You can download the full archive (Demo, Codifier, Specification) from sites like 4EGE or OnlyEGE .
These documents list all the topics that will be tested and explain how the exam is organized.
SDAM GIA (Reshu OGE) allows you to solve individual tasks by topic or complete full randomly generated variants with instant feedback.