There is often a case of what people are supposed to do and what they actually do.
Similarly, there is what people are taught, and what people actually do (or remember!).
I very seldom dive from boats where there is a DM. Normally for me this is only on the rare occasions I squeeze a dive in when working in the USA, or something like a Red Sea Trip.
Most of my diving is in or around the British Isles (England, Scotland, Wales, Ireland and Northern Ireland). As a general rule we book the complete boat as a group, or guest on a boat. The dive managing will be down to us. The boat briefing down to the skipper (Captain), the site briefing normally down to the skipper (Captain). The dive parameters by agreement, generally this is how long we will be in the water, and how much decompression we will run, and if its DSMB's on the ascent, shotline etc.
If we have newer divers on the boat we will tell them to do a buddy check, plus they will get a dive briefing from their experienced buddy (instructor) . If we have newer divers diving together, they will get a briefing from us, and they will have to confirm their dive plan with our dive manager. If I'm with a new diver, they will get a dive briefing, and we will be doing buddy checks. Divers I don't know will get the same treatment as a new diver, until I have a feel for them, then I will cut them some slack.
Divers I do know, we will discuss the dive and our preferences, and modify to the most conservative. Truthfully, we might not do our buddy checks in front of each other, doing our own personal checks.This is almost always followed by the question,Gas On?, before we approach the boat exit point.
If I don't trust someone they will get a buddy check, no matter how many dives they have, trust is earned and easily lost.
We teach and dive as buddy pairs, not as groups. As a general rule, the visibility is not reliable enough or conditions benign enough to assume you can dive as a group. Hence the slow escalation of dive responsibility for new divers, buddy very experienced, buddy similar experience on a 'easy' (known) site with over site on the dive plan by the dive manager, to a buddy pair planning their own dive and managing their own dive. For us, if a pair of newer divers are diving together and choose to abort the dive, its a good sign for us that they are learning, taking the dive seriously and have the good sense that its better to be back on the boat regretting cutting the dive short, than in the water wish they where on the boat.
Ultimately, dive conduct is your decision. Is the dive suitable, is it safe, are the conditions safe, are you capable of safely doing the proposed dive, are you happy to dive with your buddy, the guide, the boat.
The biggest leap is going from student, where you do what you are told without question, or because the instructor told you to do it, and pass responsibility for your safety to the instructor (which you should never do anyway). To taking personal responsibility for your own safety. - that's when you are a qualified diver (the C -card has nothing to do with it).
Gareth