SWAP doesnt necessarilly need to be listed in fstab since systemd
(and neither does tmpfs)
But can be put there if you want to configure specific options.
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.
Years ago when I installed manjaro, I also chose manual partitioning, I had to add my swap partition myself.
This is an example of what mine looks like: