OTOH once you are in estar's high background detail 'Majestic Wilderlands' (or whatever) then how would the players do that? They can't easily just invent an old boarded over basement window everyone forgot about, for example. Nor can they simply invent a judge to be bribed to explain the +1 they got on some check because they own the Warehouse District. All of that COULD be sorted out, but it has to be sorted out by the GM,
It sorted like if you were there as the character, you check the condition of the warehouses' basement windows before. Or you take your chances. You ask around to see if there is a bribable judge, bribe them and use them to further your goal.
to explain the +1 they got on some check because they own the Warehouse District.
They don't need to explain why the +1 because already die something as their character to earn that +1. At the level of specific action it expressed by having the players describe what it is their character is doing. And if it warrants a modifier then it get a modifier. And in majority of the case circumstances modifier is already defined in the rules.
because nobody else has permission to introduce any fiction, and nobody else has his (what must be) 1000's and 1000's of pages of notes on everything under the Sun!
I have a lot of notes but not 1,000s of pages. For what I haven't detailed at that moment, I will turn what been described about the setting. The odds are significant in a run down neighborhood that one of the basement are broken. So I make a roll reflecting those odds and those are the result the players and I deal with. If it exists the players caught a break, if it doesn't then the players have to deal the situation with that detail.
If they want create a advantageous situation then they have to create as if they were there. The challenge is part of the appeal. And it not for everyone like any specific niche within this hobby. But it absolutely doesn't require 1,000s pages of note. It does require some work and some planning like access to a decent set of random tables for the setting.
In a more subtle way, how fitting are all of those details to the execution of a given sort of activity (IE gang building in this case) to the established fiction, which is so dense that there may well be no area in the whole setting where you won't run into endless obstacles that thwart any such effort?
Life is vast and full of possibilities even in a medieval hamlet. What you are calling obstacles are not obstacles. The vast majority will be neutral towards the PCs. Some will be allied or positive circumstances, other will be enemies or negative circumstances. I would say a referee running a campaign the way I do make everything negative or everything positive or even everything neutral is not doing a good job of making a compelling setting. Places have a mix. Exactly what there is discoverable. With that information one can formulate a plan to further one's goals.
Take my video for example. The inciting incident is that you are camped on the side of the road at night. You see another campfire, you hear a scream what do you do?
Do you get involved? Do you ignore what going on? Which side you join when youd.
If you opt to get involved you will find bandits attacking two individual. One obviously a young man who is a peasant, the other a young woman who obviously a noble's daughter.
Now I have run this over half a dozen time. Every time the group decides to rescue the pair and fight the bandits. But I am prepared if they decide to join the bandits. Or to ignore the situation and let the bandits have their way with the couple.
Most assume that I mean for the group to rescue the couple. That it is my preferred outcome. Well I don't have a preferred outcome. I tell players do what you want to as a character. I will roll with it. Join the bandit and make the couple the party's prisoners, OK. Ignore what going on, OK.
But I find that everybody wants to be a hero and thus fight the bandits and rescue the couple. And they learn that the couple is running away and that the lady's father is a knight and that he doesn't want the two to marry.
And this incident is the first major point of divergence in how the different groups handle the situation. A slight majority will take the couple back to the village. All of them came up with a story to get the peasant boy out of trouble. Half of them succeed and other half didn't because father is a knight who is the bailiff (administrator) of the village and major naughty word. Some group are good at talking down naughty words some are not. The last group did it succeeded because one of them was a paladin of the major faith of the region and she milked it for all it worth and intimidated the naughty word out of the knight.
Keep in mind I let players roll up characters for my one-shot so I no idea beforehand this was going to happen. But she picked up on the social implication of the background details I gave her on paladins right away.
Other groups will find a hiding place in the woods (there is an abandoned cottage that not far from the village that they can find) and stash the couple there while they deal with their mission from the bishop and figure the deal with the girl's father the knight.
So yes the players are not creating the fiction. But I breathed enough life into the setting to bring it to life so they don't need to in order to experience it and find opportunities to advance their own goals. And because the place has it own life it has it own troubles that the players find themselves in the midst of.
Not something they anticipated as players. It not a fiction they created to experience. They literally had no idea of the complications they were about to face. But the part of the fun, dealing with the unexpected. Then finding a way to resolve it.
I admit there is a chance they could ignore the complications. The conflict between the knight and the villagers, the plots of the Russet Lord a winter faerie lord seeking to recreate one of the stories that gave him birth that will result in the destruction of the village. The slothful monk who habitually late on the tithe. The players could march right in, drop off the couple, resolve getting the tithe, and march right out*.
But the reason I work on this particular adventure is that like my Scourge of the Demon Wolf I found that the overall situation was compelling to a broad range of players. That given human nature, players decide on their own to deal with it in a way that they find fun. And to me that how the best sandbox campaigns work. I don't have do anything keep the campaign going. They want to continue, to climb the next hill, talk to the next NPCs.
And all this approach is neither better or worse than any of the other in this. But it does offer quite a bit of player agency.
*So this happened not in my Russet Lord adventure yet, but in my Scourge of the Demon Wolf adventure there is a red herring where the players can encounter a group of bandit pretending to be wolves. Most group don't think the bandits are the cause of the problem that they were sent to investigate. But one group did, they were positive that they solved the mystery. And this was after 1 hour in a four hour time slot at a convention.
Luckily they encountered the bandits well before reaching the village so never went there to talk to them. When they returned to the baron that sent them out, he asked OK are they going to bring in my harvest. The players realized they didn't complete that part of their task. So headed back out. And quickly found out that the bandit were not the sole cause of the wolf attacks.