That's three separate questions. First, the kernel....
Any operating system is just a collection of programs working together. The kernel is the program responsible for coordinating the activities of all the other programs. Think of it as the manager of a store.
Bloatware is preinstalled software that has little usefulness, typically as some kind "store" or ad. Mostly used to extort money, or maintain draconic control over the system.
The bootloader is a tiny, primitive operating system, it's job is to check the hardware at startup and load up the main operating system.