Not exactly, blocks are 4KiB so the vast majority of the CRC32C's apply to 4KiB block size; the metadata uses 16KiB nodes and those have their own checksum also

Not exactly, blocks are 4KiB so the vast majority of the CRC32C's apply to 4KiB block size; the metadata uses 16KiB nodes and those have their own checksum also. From what I see in the on disk format docs, it's a 20 byte checksum.