Rule no1:
Be friendly.
Historically cyclists greet each other to signify their help is available in an emergency like an accident or a breakdown.
Motorists used to do this in the early days.
I had an incident where a man jumped out of his car and threatened to stab me for not getting out of his way fast enough when he wanted to reverse at a junction...
( I don't think he understood how hard it is to reverse down a hill on a racing bike
)
A group of deliveroo cyclists waiting to pick up at the adjacent restaurant just stood there watching
I dont think they would say hello.