For high-ranking police officers, various trade associations naturally come to them with bribes, while frontline officers often demand money themselves.
At least in Arthur's experience, when he served at Scotland Yard, he handled quite a few cases of frontline patrolmen extorting cash.
The targets of such extortion are mostly those engaged in shady industries who have been caught holding incriminating evidence.
Of course, bribing police officers isn't always an option. Apart from the two types with exceptionally high moral standards and extreme greed, whether most officers take bribes largely depends on whether they're looking for promotion or quick wealth at the moment.
Arthur was just about to chat with Victor, but unexpectedly a low English conversation came from outside the door.