Here's how I approach it:
When I prep a sandbox campaign, I start by building a setting that would make sense even without the players. I populate it with factions, locations, conflicts, and situations that are plausible given the premises of the setting, not constructed around the assumption that players will interact with any specific element. This principle is crucial to avoiding railroading.
I don't see how it has any effect at all on railroading. Just because you aren't assuming the PCs will interact with it, doesn't mean you cannot be nailing down one and only one valid path forward. Design a religion that is utterly unpersuadable--by anyone, PC or not. Design a marauding horde, a reasonable thing in almost any fantasy setting, which reasonably besieges towns. Said thing
can then be used to control player motions in various ways.
Populating the world with stuff without considering the PCs doesn't do anything to start or stop railroading.
This material forms part of my "Bag of Stuff": a body of prepared details that give players meaningful choices once play begins. But none of it demands players act a certain way or follow a predetermined path.
See above. It absolutely can,
when they then do interact with it. And, as DM in control of what "makes sense" etc., you can ensure that such interaction eventually happens--indeed, even if you very specifically created these things without any thoughts whatsoever of the PCs or how they could potentially interact with them, an enormous chunk of that 'Bag of Stuff" can then be used as tools to control their behavior. Which is the whole point. This isn't railroad prevention. It merely furnishes setting elements. Those elements can then be used in whatever way any DM likes--including to railroad, even if not a single thought was given to railroading in their creation.
One part of my approach that often gets overlooked is how this all manifests at the table: through first-person roleplaying. I don't just describe the world abstractly. The world comes alive when I roleplay NPCs in the first person, and players respond likewise.
I genuinely don't understand how this is relevant.
This matters in two ways to the points you raise:
1) It grounds later discussion with players. If a question arises, we can review how I roleplayed the NPC based on the established notes and setting facts, for example, whether my portrayal of Thranduil in the Rhovanion sourcebook for AiME was consistent with the material.
I don't see how this responds to what I said in the slightest. Like I'm baffled as to why you even mention it. I cannot mount a meaningful response beyond this, because I literally don't understand any relevance of this to the thing you say it is relevant to.
2) First-person roleplaying ensures players are presented with environment where they have the full range of options that exist in real interaction. This makes it virtually impossible for me to control player choices or railroad them into specific outcomes.
It does no such thing. Again: If there's a marauding horde besieging the walls of the Merchant Republic of Aiztenev, while plague scours the people within, then that goes nearly all of the way toward controlling the players' actions: they either stay in the city and risk death, or leave by ship since the marauding horde has only limited ability to blockade the ports.
First-person narration does nothing whatsoever to prevent such an intersection of pre-established game pieces from narrowing the players' options to either one and only one path (a full, unequivocal railroad, though not the most extreme possible railroad), or to a finite set of pre-approved DM-authored options (what I call a CYOA, which is still a railroad, it's just got forks.)
This process is reinforced by my emphasis on referee impartiality. The referee's job is not to shepherd players to a "correct" outcome, but to present a living world, adjudicate fairly, and let consequences unfold naturally, even when it surprises me.
So...I'm gonna level with you, this reads as "the process is reinforced by my solemn promise not to railroad." Which really makes the argument seem both circular and superfluous, even before the things I've said above.