To mount a Btrfs file system automatically at system startup using the /etc/fstab file, you need to add an entry for the file system in the following format:
UUID=<UUID of the Btrfs file system> <mount point> btrfs defaults,subvol=<subvolume name> 0 0
Here, replace <UUID of the Btrfs file system> with the unique identifier of the Btrfs file system, <mount point> with the desired mount point, and <subvolume name> with the name of the subvolume you want to mount.
For example, to mount a Btrfs file system with the UUID 0ccb00d7-e045-4f70-9c09-d64cbf7b8e8b at the /mnt/btrfs mount point with the default subvolume, the entry in /etc/fstab would look like this:
UUID=0ccb00d7-e045-4f70-9c09-d64cbf7b8e8b /mnt/btrfs btrfs defaults,subvol=@ 0 0