It doesn't help if you post multiple topics with related questions; it is not pleasant to follow links to find out what you want to do.
You get a better chance of answers if you state which display you have and which Arduino board you use.
All code extracts should be "complete", should allow to be compiled. It is not pleasant to have to study your code to find out what you want to achieve.
With Arduino MEGA or any 5V Arduino, use at least series resistors on data lines.
Read the README.MD file of the library you use, and for GxEPD2_AVR follow the link to Oliver's explanation of paged drawing and picture loop.
The do{…} while (display.nextPage()); loop is needed so that all pages that may get changed by the contained code get drawn to the controller memory and updated to the screen in the last nextPage call.