Not at all! The idea of a "boot sector" is spefific to the DOS systems, based on x86 and BIOS.
On x86 and x64, the three boot loaders most often used are, AFAICT, LILO, GRUB and SysLinux.
However, the kernel doesn't care about how it gets into memory; something needs to load it at the right address, and jump to it. Discovery of how the system is configured is typically hard-coded on embedded systems (like the Pi), or goes through a mechanism like AHCI (on x86 and friends).