Filesystems are usually extended by custom ioctls beyond the standard system call interface to let user applications access the advanced features. They’re low level and the following list gives only an overview of the capabilities or a command if available:
reverse lookup, from file offset to inode, as command btrfs inspect-internal logical-resolve
resolve inode number to list of names, as command btrfs inspect-internal inode-resolve
tree search, given a key range and tree id, lookup and return all b-tree items found in that range, basically all metadata at your hand but you need to know what to do with them, the ioctl is privileged as it has full access to all filesystem metadata
informative, about devices, space allocation or the whole filesystem, many of which is also exported in
query/set a subset of features on a mounted filesystem
Programming documentaion of the ioctls is in the manual page btrfs-ioctl(2).