For example lets say we have a very simple, but bespoke Quest where someone has to go to the Western Watchtower, and report back to the Jarl about the supposed dragon sighting in the area.
The bespoke Acts for this would be:
Consult the Soldier
Meet up with the Scouting Party
Travel to the Western Watchtower
Investigate the Area
Slay the Dragon
Report to the Jarl
As you'll notice, each one has a clear "Verb" that signifies the overall idea of that Act, and this is more or less how the Generic Quests will work.
Now, lets say the players never get involved. This quest would trigger immediately after returning the Dragonstone from Bleak Falls Barrow, so lets say the players just bamf out and aren't immediately shuffled into the new Quest.
How this would be handled would be Time Based. The Keeper, as part of their Prep, would have already rolled some dice to determine how long each Act takes before it progresses into the next. The Blocks for the Quest would note the dice to roll and the Time denomination; in this case, we're talking minutes for pretty much every single Act, and so we'll have the dice be 2d20. 40 minutes max between each Act, which seems reasonable even for a more realistically sized Skyrim.
So, if the Players bamf out, they'll still be playing, and so time will progress as they do so. As this Quest in particular happens within a matter of an hour or two, The Keeper will probably be tracking this just using the Blocks. For more longer term Quests that could have days, weeks, or even months in between Acts, they will use a Calendar to track the Quest and mark that its advanced to the next Act. (And notate when the next Act triggers, if they didn't already predetermine it)
For Bespoke quests, what occurs as a part of each Act will be pre-written, meaning that regardless of the level of player involvement, the events of the Act that aren't affected by the Players will simply backfill into the Canon when the Act triggers. (Generic Quests are more open to interpretation and improvisation in this regard)
Now, as to what happens. The Players have Bamf'd, so no one had technically taken up the Quest. Lets say that Irileth, the Dunmer Housecarl to the Jarl, is what's called a Keeper Player Character, or KPC. KPCs are essentially a variant NPC who, in collective with all the other KPCs of their Region/City, basically form a systemically created "Culture" for these places. But in terms of Quests, depending on the KPC's personality, they will have a chance to take up any Quests that are pending in these areas.
Because this is a bespoke Quest, and because this specific KPC was already a part of the intended Scouting Party, we'll say that she takes up the Quest and there's no check involved on the Keepers part to see if they or someone else does it.
From there, each Act continues, as its obvious at this point what I'm talking about, we know what happens. The scouting party meets up, they travel to the Watchtower unimpeded, and only have seconds to try and investigate the area before they're beset by the Dragon.
Personally, I don't think they'd have had a problem taking down the dragon eventually, so we'll say that Irileth eventually lands the killing blow on the Dragon.
From here, obviously, the overall Questline this Quest is a part of supposes that the Player (or one of them) is the Dragonborn, and this likely would have been predetermined when they all created their characters. So while we could try to say that Irileth becomes the DB, lets say for this example she doesn't.
So, now comes whether or not the Players opt to get involved.
At each stage, the time to trigger the next Act serves a second purpose in giving the Players a, relatively, broad window to intervene or, at minimum, witness the Quest in action.
Lets say, then, that this whole quest happens, but then the Players happen to be travelling back through the area and come up on the Dragons corpse. The Quest would, in its reference material, say what happens in this sort of circumstance.
The people of Whiterun started to carve up the Dragon, but struggle with it, and as the Players come upon the scene they'll be witness to, lets say, the town beggars being put to work trying to cut into the dragon's hide.
As soon as the intended Dragonborn approaches, and lets say, touches the Dragon, cue them absorbing its soul, and then this triggers the whole Greybeard thing.
The reference material would also say, in this instance, to re-open the original Quest's last Act, and have the Players report to the Jarl.
Each Act would have guidance like this; for example, if the players crash Irileth reporting to the Jarl, this might open a new kind of Generic Quest to Harvest Dragonbones from the Dragon. And it can go on and on like that.