Let me use the translator here, although if there are strange glitches it will be its fault 
btrfs is a complex file system. Maybe if you at least have experience with LVM (logical volume management) it is easier not to get stuck in misunderstandings, but without that it is usually not so easy to understand it.
You always have a physical volume. Let’s say a disk that the kernel mounts as /dev/sda. For simplicity, we are not even going to partition it and deploy btrfs on it. It’s not that you have a partition: the entire disk IS that partition. And you mount it as the root of the system. Let’s forget EFI partitions, boot and so on.
In this case, /dev/sda is the physical volume, and / is a volume created and mounted on the physical volume. It doesn’t matter whether you call it a “subvolume”, and there is another reason: each snapshot is a subvolume in its own right and can be mounted at any time. But let’s put this aside for a while. Let’s go back to the root /. According to the usual file hierarchy, we will have file systems such as /etc, /usr/local, /usr, /var, /home. Let’s stop at the latter. There we not only save our user configurations, but all the files. In particular, those that we download and save, whether large or small. So if we take snapshots of our system, it is very likely that we want /home to be left out, because otherwise the snapshots would be too large to be useful because they would eat up our disk space very quickly. So we tell the system “hey, don’t take snapshots of /home”, and we do that by mounting a specific subvolume for /home. But it is important to understand here that the physical space for / and /home is the same. If you have 900 GB free in / you will have them in /home, and if you have 20 GB free in /home you will have them in /. And if you run out of space, you either delete files or you will have to add more physical space to the volume (subvolume) /
Although you can use subvolumes for more things, the truth is that preventing them from taking up space in snapshots is probably the most common thing to do. The confusion comes from the fact that from btrfs’ point of view, everything is a subvolume. For example, the location of the snapshots themselves. That is what makes it possible for you to ask the system to mount a snapshot taken two months ago as / and to boot the system like this. So you can think of what you mount on / as “the main subvolume” and the others as “subvolumes within the original subvolume” and it might be easier to understand it that way. Curiously, it is how you would expect it to work: if you do not use subvolumes, you do not expect to have to distribute space between /, /var, /usr… well with subvolumes this is still the case: you have everything or nothing.
That said, one of the cool things about btrfs is that you can always resize the main subvolume. You have a Windows computer to which you have added Linux to a partition at the end of the disk and you want to incorporate the space that Windows occupied: easy, there are two ways to do it: delete the partitions, create one and add it to / and the other consists of simply incorporate the partitions a/as-is. You can add or remove partitions or even entire disks (physical space) to/regardless of their physical location. You can have, for example, three partitions of one disk and two full disks. In this case, your “volume” (or main subvolume) would be made up of:
- /dev/sda2 (1GB)
- /dev/sda3 (50GB)
- /dev/sdb (1000GB)
- /dev/sdc (2000 GB)
That would be a 3051 GB btrfs volume (you could call it, for example, root-osuse) and you could mount it as /, and as options you can tell it to create subvolumes for /var/log, /usr/local and /home. In that case, you would have 3051 GB available for the entire system, be it /, /home, /var/log or /usr/log. It may be that you work with multimedia projects and suddenly in /home/tux/projects/ you have files that occupy 3000 GB and in that case you will quickly run out of space on the system. But no problem: you can add a new 4000 GB /dev/sdd disk and add it to “osuse-root” and now you will have a total space of 7051 GB for the entire system and about 4000 GB free (plus whatever you still had free ).
Ok, as far as I know you can’t give a name like osuse-root to the main volume and I’ve simply used that rhetorical device because I think it’s easier to explain it that way. Now go to YaST → System → Partitioner and take a look, look at the left column. Among other things, you will have two related entries: one is disks and another is btrfs. Check the difference between the two. That’s the way it works. In CLI, to indicate btrfs that you need change the main volume, you use “filesystem [mount-point]”.
Just kidding. I like to keep things clean and tidy so I’ll keep deleting. And when I get free time, I’ll dive into some of those little Linux projects I’ve put on my todo list.