What does a | b do in C (bitwise OR)?

Bitwise OR sets each result bit to 1 where either input has a 1 — so 0x69 | 0x55 = 0x7D; it's the go-to operator for turning flag bits on.

0x69 | 0x55  →  0x7D

  01101001  (0x69)
| 01010101  (0x55)