During installation the guide tells you to run a program, which generates an fstab based on current mounts. So by default it will configure /tmp the same way it’s configured in the live cd.

Actually the installation guide for arch asks you to use `genfstab -U /mnt >> /mnt/etc/fstab` which basically copies over whatever was mounted at the live environment (taking off the /mnt), I'm pretty sure tmpfs for /tmp was there by default last time I installed.