Linkers
Games are written to the cartridge with a device called "linker". Depending on the brand of flash cartridge, the linker either connects to the GBA's serial link port and writes to the cartridge through the GBA or connects to a mini-USB slot on the cartridge itself and writes directly to the flash cartridge. These linkers usually connect to a PC through a USB or parallel plug on the other end. Most linkers that connect to the GBA link slot are capable of copying ROM information from commercial Game Boy Advance software cartridges. Some more recent flash cartridges use digital media cards (SD,MMC,CF, etc.) in which files are placed via a memory card reader.
Read more about this topic: Game Boy Advance Flash Cartridge