Depending on the installed bootloader and OS, there might exist more boot sectors on partitions too. In case of Windows, executable code in MBR finds Active Partition or Boot Partition (Windows calls it System Partition) in Partition Table (also part of the MBR) and executes that partition's boot sector (VBR) which contains BootMgr code. By now VBR is large enough to understand filesystem structure, so it executes \bootmgr file (placed in root of the Active Partition), which reads its configuration from BCD file (usually \Boot\BCD) and runs winload.exe (or shows a selection menu if BCD contains multiple entries), which runs Windows kernel. See more details here.