The command you're looking for is btrfs send. See man btrfs-send.

I know of at least one tool, btrbk, which automates both automatic periodic snapshots and incremental sync, but here's an example manual process so you can know the basic idea. Run all this in a root shell or sudo.

As initial setup:

• Create a btrfs filesystem on the sender drive and another on the receiver drive. No need to link them or sync anything yet, although the receiver's filesystem does need to be large enough to actually accept your syncs.
• Use btrfs subvolume create /mnt/mybtrfs/stuff on the sender, substituting the actual mount point of your btrfs filesystem and the name you want to use for a subvolume under it.
• Put all the data you care about inside that subvolume. You can mount the filesystem with a mount option like -o subvol=stuff if you want to treat the subvolume as its own separate mount from its parent.
• Make a snapshot of that subvolume. Name it whatever you want, but something simple and consistent is probably best. Something like mkdir /mnt/mybtrfs/snapshots; btrfs subvolume snapshot /mnt/mybtrfs/stuff /mnt/mybtrfs/snapshots/stuff-20250511.
• If the receiver is a separate computer, make sure it's booted up and running an SSH server. If you're sending to another drive on the same system, make sure it's connected and mounted.
• Send/copy the entire contents of the snapshot with a command like btrfs send /mnt/mybtrfs/snapshots/stuff-20250511 | btrfs receive /mnt/backup. You can run btrfs receive through SSH if the receiver is a separate system.

For incremental syncs after that:

• Make another separate snapshot and make sure not to delete or erase the previous one: btrfs subvolume snapshot /mnt/mybtrfs/stuff /mnt/mybtrfs/snapshots/stuff-20250518.
• Use another send command, this time using the -p option to specify a subvolume of the last successful sync to make it incremental. btrfs send -p /mnt/mybtrfs/snapshots/stuff-20250511 /mnt/mybtrfs/snapshots/stuff-20250518 | btrfs receive /mnt/backup.

If you want to script a process like this, make sure the receiver stores the name of the latest synced snapshot somewhere only after the receive completes successfully, so that you aren't trying to do incremental syncs based on a parent that didn't finish syncing.