One correction, you don’t use adb to flash custom ROMs, it’s just for interacting with a working Android device.
For flashing you use other tools which can write partition images to the phone’s partitions and know the format that the vendor packages those images and the partition structure on the phone. fastboot is a standard tool for that but some manufacturers don’t follow standards and there are other tools made specifically for them.
Also, TWRP is not a bootloader, it’s a recovery. The recovery is sort of a cross between a PC BIOS and a rescue disc, it’s a mini-OS you can boot into and perform maintenance operations. Phones already come with a recovery built-in but it’s super-basic so most people prefer a TWRP-made recovery because it has much more features.