it is correct.

<<3, 0, 0, 0>> is a bitstring, encoded as a 4 bytes it is 0x03 0x00 0x00 0x00 and as a 32bit integer it becomes 50331648.

50331648 & 0x01 is zero, because the last byte of the integer is not set (the fourth zero in the four bytes sequence)

0x03 0x00 0x00 0x01 (50331649) would give you 1.

iex(11)> <<chunk::little-unsigned-size(31), is_first_chunk::little-unsigned-size(1)>> = <<3, 0, 0, 1>>
<<3, 0, 0, 1>>
iex(12)> is_first_chunk
1

iex(17)> <<chunkX::32>> = <<3, 0, 0, 1>>
<<3, 0, 0, 1>>
iex(18)> require Bitwise
Bitwise
iex(19)> Bitwise.&&&(chunkX, 0x01)      
1
iex(20)> chunkX
50331649

iex(21)> <<chunkX::32>> = <<3, 0, 0, 0>>
<<3, 0, 0, 0>>
iex(22)> Bitwise.&&&(chunkX, 0x01)      
0
iex(23)> chunkX                         
50331648

iex(24)> Bitwise.>>>(chunkX, 1)   
25165824 <-- chunk

EDIT: you can convert the integer back to a bitstring with

iex(25)> :binary.encode_unsigned(50331648)
<<3, 0, 0, 0>>
iex(26)> :binary.encode_unsigned(50331649)
<<3, 0, 0, 1>>

Yup, I’m workin on VelocyStream (ArangoDB).

With your insight, it became clear. To encode for an outgoing chunk:

chunk_x =
  <<chunk::31, is_first_chunk::1>>
  |> :binary.decode_unsigned(:little)
  |> :binary.encode_unsigned(:big)

and decode from an incoming chunk:

<<chunk::31, is_first_chunk::1>> =
  chunk_x
  |> :binary.decode_unsigned(:big)
  |> :binary.encode_unsigned(:little)

I really appreciate it, thank you.