And the lead writer? What is his job?
And in Mass Effect 's case, what do you think Casey Hudson was doing? And the producers? Why do you think the writers spend so much time talking?
First of all, I can only say what I've heard but I'll try to piece it together how I've understood it:
Lead writer is responsible for the major story arc for the game... or well, the main plot so in Mac's case and ME3 that would be the premise. He had part in deciding that the Genophage should be resolved and the Geth/Quarian war should be resolved and he was also the guy who handpicked which senior writers should tackle each subplot. He himself wrote all major missions in between those arcs for the most part. I'm pretty sure Mac mainly wrote Vancouver, Mars, Cerberus Coup, Thessia, London and the endgame.
Basically each writer, both seniors and leads write individual missions and characters but the lead ultimately decides what the overarching plot should be and as Mac Walters called it in his interviews "The narrative backbone or emotional beats of the story".
But all writers touched upon every missions either throughout or in peer-reviews. Almost every mission in Mass Effect 3 - and this goes for most stuff in DA:I and past Bioware games as well too - were written by its writer but then once characters had to be incorporated their individual writers would write that dialogue or give it a thumbs up if the writer of the mission had given said characters a good enough line and furthermore they'd look at the entire mission and give constructive criticism on it. After that they'd either give it green light or re-iterate it which happens a lot in every Bioware game.
For example Cerberus HQ was mostly written by Patrick Weekes, as well as all the video logs, but after writing the first line of dialogue for TIM Mac took over and I guess further peer reviewing was done on it... or not, cuz there are some oddities at the end: TIM rambles and Shepard and Co. just leave the VI there in a very abrupt way.
Also, David Gaider has said multiple times that he has a vision for where Dragon Age will go in the end and he hopes they can still get to that point. The series, from the start, was planned with a "5-game plan" so we're just over halfway, and with DA:I dabbling quite a bit around in faith and gods I think I can see where the series might end up thematically already and it makes me excited.