Generated unit files are not automatically activated by systemd. There's nothing special about them as far as systemd is concerned. Each individual generator has to explicitly create symbolic links that connect a generated unit to a target, so that activating the target activates the generated unit via a dependency in the normal way.
This takes advantage of the fact that not all connections between units are expressed inside the unit files. Wants and Requires dependencies can be expressed with symbolic link farms in
*.requires/ subdirectories. And those symbolic link farms encompass subdirectories of
/run/systemd/ amongst other things.
In other words: Rather than write out a unit with
WantedBy=local-fs.target and then have to explicitly invoke
sytemctl enable to make the symbolic link (which is what enabling does), the generator short-circuits the process and makes the symbolic link itself. And it makes it in an ephemeral place, which
systemctl enable normally would not, thereby preventing the symbolic link from continuing to exist after the next shutdown and confusing the next startup.
Specifically, you'll find on your system that
/run/systemd/generator/boot.mount is symbolically linked from
systemd-fstab-generator created this symbolic link, and it makes
boot.mount wanted by
local-fs.target. The generator is run early in the bootstrap before the system gets around to activating
local-fs.target, which means that when it does get around to activating
local-fs.target the generated dependency is there to be followed.