Generate the fstab

The new system needs to know how to mount its own filesystems at boot. Right now that knowledge exists only in the live system’s mount table – which is exactly what genfstab reads:

Run this on the live system:

genfstab -U /mnt >> /mnt/etc/fstab

-U writes UUIDs rather than device paths. This matters more than it looks: /dev/sda is assigned in whatever order the kernel happens to find disks, so adding a second drive later can rename the first one and leave a machine that no longer boots. A UUID belongs to the filesystem and never moves.

Append, do not overwrite -- and only once

Note the >>. pacstrap already created /mnt/etc/fstab with a header comment in it, and a single > would replace it.

More importantly, running this command twice appends a second copy of every entry. Duplicate mounts are a confusing failure at boot. If you are unsure whether you already ran it, read the file before running it again – not after.

Read what it wrote

This file is worth actually reading, because a mistake here is a machine that does not come up:

[bash]: Run this on your workstation:

cat /mnt/etc/fstab

(stdout)

# /dev/nvme0n1p2 LABEL=arch
UUID=e1445380-51a2-449d-ad82-8ac3286cbfb6	/         	btrfs     	rw,noatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvol=/@	0 0

# /dev/nvme0n1p2 LABEL=arch
UUID=e1445380-51a2-449d-ad82-8ac3286cbfb6	/home     	btrfs     	rw,noatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvol=/@home	0 0

# /dev/nvme0n1p2 LABEL=arch
UUID=e1445380-51a2-449d-ad82-8ac3286cbfb6	/.snapshots	btrfs     	rw,noatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvol=/@snapshots	0 0

# /dev/nvme0n1p2 LABEL=arch
UUID=e1445380-51a2-449d-ad82-8ac3286cbfb6	/var/log  	btrfs     	rw,noatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvol=/@log	0 0

# /dev/nvme0n1p2 LABEL=arch
UUID=e1445380-51a2-449d-ad82-8ac3286cbfb6	/var/cache/pacman/pkg	btrfs     	rw,noatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvol=/@pkg	0 0

# /dev/nvme0n1p2 LABEL=arch
UUID=e1445380-51a2-449d-ad82-8ac3286cbfb6	/swap     	btrfs     	rw,noatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvol=/@swap	0 0

# /dev/nvme0n1p1 LABEL=ESP
UUID=4B65-AADD      	/boot     	vfat      	rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,errors=remount-ro	0 2

/swap/swapfile      	none      	swap      	defaults  	0 0

Three things to check:

Every btrfs line carries the same UUID, and they differ only by subvol. That is one filesystem mounted six times, which is what subvolumes are.
The boot partition has its own UUID and is vfat. Its UUID is short – FAT volume IDs are 32-bit, so they look like A1B2-C3D4 rather than the long form used by btrfs.
The swapfile has a line if you created one.

Options you did not ask for

genfstab records the mount options the kernel is actually using, not the ones you typed, so you will see extras like ssd and space_cache=v2 that btrfs selected for itself. That is correct and should be left alone.

No swap line?

genfstab finds swap by looking at what is currently active, not at what exists on disk. If you created a swapfile but never ran swapon, there will be no swap line here and the installed system will come up without swap, silently.

Fix it by running swapon /mnt/swap/swapfile and generating the file again – after deleting the entries you already wrote.