# In the case of Chloe (2022-11-24):
mount -t btrfs -o noatime,subvol=rootfs /dev/sda4 /mnt/p4rootfs
mount -t btrfs -o noatime,subvol=home /dev/sda4 /mnt/p4rootfs/home
mount -t btrfs -o noatime,subvol=usr /dev/sda4 /mnt/p4rootfs/usr
mount -t btrfs -o noatime,subvol=var /dev/sda4 /mnt/p4rootfs/var
# etc

# A site suggests using a variable for base mount point:
chr=/mnt/p4rootfs
mount -t btrfs -o noatime,subvol=rootfs /dev/sda4 ${chr}
mount -t btrfs -o noatime,subvol=home /dev/sda4 ${chr}/home
mount -t btrfs -o noatime,subvol=usr /dev/sda4 ${chr}/usr
mount -t btrfs -o noatime,subvol=var /dev/sda4 ${chr}/var
mount -t proc proc ${chr}/proc
mount -t sysfs sys ${chr}/sys
mount -o bind /dev ${chr}/dev
mount -t devpts pts ${chr}/dev/pts
chroot ${chr}

# do whatever
exit # from chroot

umount ${chr}/dev/pts
umount ${chr}/dev
umount ${chr}/sys
umount ${chr}/proc
umount ${chr}/var
umount ${chr}/usr
umount ${chr}/home
umount ${chr}