How to properly mount an btrfs device with subvolumes (temporarily)

a few days ago my main pc died, it had tumbleweed installed (one hard disc with two partitions: 1. btrfs for / [and subvolumes] and 2. xfs for /home).
I bought some new hardware and build a new pc (also with tumbleweed but on a new hard disc). Now i want to copy some old files from the old hd to the new system,
so i plugged the device in and restarted the system, as expected i the two partitions are listed an dolphin and i can access them by just clicking on them, which automounts them to /run/media/USERNAME/<cryptic device id>.

This works great for the xfs /home partition but the btrfs partiton is missing all of its subvolumes (/opt /usr/local etc. are all empty). I realised that a btrfs partition requires a lot of more entrys in /etc/fstab
but i hope that there is an easier way to mount it

i think it would be ok to manually mount the subvolumes but i cannot find information about the syntax, i guess it is something like this:

mount -t btrfs /dev/sdx1 /run/media/USERNAME/<cryptic device id>/subdir -o subvol=@/subdir

from reading the /etc/fstab i guess that the @ stands for the current or selected device right?

I used something like this in a script:

## SUBVOL=`sed -n 's/.*subvol=\(^,       ]*\).*/\1/p' $MOUNTWHERE/etc/fstab`
## for sv in $SUBVOL
## do
##      mount -o subvol=$sv $ROOTDEV $MOUNTWHERE/$sv
## done

The lines that begin “##” are now comments. I commented them out when I switched back to “ext4”.

(note that switching back required a re-install).