GRUB2 (https://www.gnu.org/software/grub) has the most advanced support of booting from BTRFS with respect to features.
U-Boot (https://www.denx.de/wiki/U-Boot/) has decent support for booting but not all BTRFS features are implemented, check the documentation.
In general, the first 1MiB on each device is unused with the exception of primary superblock that is on the offset 64KiB and spans 4KiB. The rest can be freely used by bootloaders or for other system information. Note that booting from a filesystem on :doc:`zoned device<Zoned-mode>` is not supported.