

Companies used to (and some still) transfer profits to shareholders by paying periodic dividends. The stock buyback transfers profits to shareholders by raising the stock price. It became popular because capital gains are taxed at a lower flat rate than dividends.
Also, dividends are taxed when they are paid, but gains are taxed when the stock gets sold. Wealthy shareholders can sit on unrealised capital gains for years or decades, pay no taxes, and still access the wealth by putting the shares up as collateral for personal loans.
Stock buybacks are certainly popular with big and wealthy investors.
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