To open Fastboot mode on a Samsung Galaxy A7 (2018) or any other Samsung device that might be referred to as "repack," you generally follow a standard set of steps. However, it's crucial to note that Samsung devices typically use Odin Mode (also known as Download Mode) instead of Fastboot mode, as Fastboot is more commonly associated with devices from other manufacturers, often utilized through Qualcomm's Emergency Download Mode (EDM) or similar.
On a Galaxy A7, developers sometimes patch the bootloader or use a custom kernel that adds Fastboot functionality alongside Download Mode. In rare cases, the repack replaces the recovery partition with a Fastboot interface.
If you want, I can provide model-specific firmware links and an exact Odin/flashing walkthrough for your Galaxy A7 model (I will need the exact model number, e.g., SM-A700F).