The excerpt from here: https://linuxhint.com/resize_a_btrfs_filesystem/
The Btrfs filesystem can be resized online (when the filesystem is mounted), but if you want to resize a partition that is added to a Btrfs filesystem, you will have to do it offline (when the filesystem is not mounted). You can grow/expand or shrink a Btrfs filesystem online and grow/expand or shrink a Btrfs partition offline.
What is the difference between a Btrfs filesystem and a Btrfs partition in this context?
What i believe is that the partition is just a space range on a disk, which is written in some partition table. And the filesystem is the data structure inside that space range. How does this fit in the context of Btrfs resizing in the mentioned article?
EDIT: what would be the reasons for rezising/shrinking a filesystem without shrinking its partition? Is it possible for two filesystems to be on the same partition? I want to online shrink Btrfs and create a new partition with the freed space, but this confuses me.