systemd activates swap partitions based on two different mechanisms. Both are executables in /usr/lib/systemd/system-generators. The generators are run on start-up and create native systemd units for mounts. The first, systemd-fstab-generator, reads the fstab to generate units, including a unit for swap. The second, systemd-gpt-auto-generator inspects the root disk to generate units. It operates on GPT disks only, and can identify swap partitions by their type GUID, see systemd#GPT partition automounting for more information.