Windows was never oriented around “just works”. That was Mac. Windows’s main selling point is that it never becomes incompatible, and that has largely stayed true. You can still to this day insert the disk for some proprietary application from Windows 2.0 and it will still install and run. Try that with another operating system!
Within section 2.1 choose only one subsection to follow. Those are all alternative bootloader options.
The bootloader subsection chosen in 2.1 on this page should match what is done in Configuring the Bootloader. The default path on that page is GRUB, which does not require any systemd components.
If following the GRUB path, follow instructions in 2.1.1 and skip the rest of 2.1. This is not at all clear in the handbook.
I believe that sys-kernel/installkernel is a utility script internal to the Gentoo project that can be configured to work with various bootloader solutions, including (optionally) systemd, and that is what this section 2.1 is talking about.
This appears to be an out of order dependency in the handbook