Location Design
Whether it is a wilderness to explore a dungeon to delve, location-focused design is about setting up a fictional play space with enough things for players to interact with to provide entertainment and enjoyment during the game. This is your standard “sandbox” type of deal. A point-crawl, a hex crawl of some kind, and again, dungeons, all fall into this when they are being made with the primary concern being exploring the fictional space.
Secondary goals might exist such as looking for a specific item or NPC, attempting to do a specific task, or even just getting experience points. However an adventuring Location should work just as well without any objective beyond “this is a place to play in”.
Location-focused adventures can often benefit from having a scenario of some kind set up beforehand, as the two work quite well naturally. A plot of some kind could also be slotted in, of course, but in most cases that will simply overshadow the exploration of the location as a goal in and of itself.
How this differs from Scenarios is that the location does not need to have a specific situation or conflict currently happening as the players enter the picture. It is quite alright for a Location adventure to begin “static” and then react and shift as a result of the actions of the players.
How it differs from a Plot is that a location does not necessarily cares about specific narrative beats happening (either in order, or even at all). If the players do not interact with a specific element of the location, that does not result in bad or subpar gaming experience.
Obvious Example: Here is a dungeon composed of 20 odd keyed rooms. It has numerous enemies, hazards, treasure and obstacles. Some of the enemies form factions and have some kind of broad-strokes predetermined views of other enemies and/or factions, which gives an avenue for the players to engage in picking a side or playing all sides against each other.
Scenario Design
A different type of adventure is setting up a specific situation, outlining clearly the sides involved and their goals and parameters for “success” in the conflict, and going from there.
For me at least this type of design is primarily tied to wargaming (miniature, tabletop or otherwise). Wargaming is about conflict resolution of some kind, and so it benefits from having a scenario which outlines the parameter of the conflict.
An important part of scenario design is that it should depict the conflict already having started, and the players already begin the game by being part of one or multiple sides, and then striving to achieve their side’s goals as best as they can. Once the conflict of the scenario has been resolved, that adventure is now done and concluded - Further scenarios might be devised as logical reactions or responses to the way this first one played out, and stringing a bunch of those in a row is what a lot of wargaming campaigns end up looking like.
How this differs from a Location adventure is that while setting up the field for the conflict is part of establishing parameters, the location and its peculiarities only matter as far as they affect the conflict or scenario in question. It does not matter if over the mountains there exist some other kingdom that has other problems, because that does not directly affect the situation of the two orc tribes battling it out in this valley.
How this differs from Plot is that a scenario might have one outcome or another be more or less likely to occur as a result of the initial setup (in a “doomed last stand” kind of deal, the player or players doing the last stand are generally expected to lose eventually) however it usually does not care for specific outcomes happening or not happening. That contrasts it with a plot-focused adventure, which often needs specific narrative beats to occur so that the plot can progress without having to result to that most dreaded of term “railroading”, in which the referee simply forcibly drags the characters onto the next step of the plot, regardless of whether that makes any sense in the situation or not.
You can’t really railroad a Scenario, because if the conflict already has a 100% certain outcome, then that conflict is not barely gameable, and therefore simply unsuited for being used to prepare a gaming session.
Obvious Example: The party and their allies must defeat an imposing Big Evil Enemy. They know the broad parameters of where that Big Evil Enemy is, and the focus of the game is resolving that conflict one way or another. Once the Big Evil Enemy is defeated, or defeats the players, or any other potential resolution to the conflict has occurred, the scenario is complete and finished.
Plot Design
Plot design is the cornerstone of Trad play, and more often than not the one done very poorly. An inevitable result of it being the most broadly done way of running and playing TTRPGs I suspect.
A key difference is that a “plot” is not a “story”. Any of these types of adventures can result in a story, as the story is simply the actual events of what happened (both within the fiction and at the table). One can not write a story adventure, because that is already complete and there is nothing in there to actually play out. So instead, what you do is write a plot.
Plot adventures are most often formulated as a series of narrative elements or beats, some (or all) of which need to be met so that the players may progress onto the next set of beats. You can't go and question the doctor until you find his card at the scene of the crime (or you hear from a witness that they saw him with the victim. Or you find his name in the victim’s day planner, etc.). The reason you can’t is that the doctor might not even have existed as an element in the game, until you found the narrative beat that lets you know he even exists.
The enjoyment of playing through a plot-focused adventure is trying to find the beats and where they lead, with an ideal one often having twists and turns to the plot that the players do not anticipate, but could have foreseen if paying close attention to what has come before hand, leaving them with a broadly conventional narrative or story after completing the adventure.
How Plot adventures differ from Scenarios is that they tend to not have as clear win or lose states as a scenario would. In a scenario each side participating has very clear goals, and there is rarely much care given to results outside of those already established before play even began. In a Plot-based adventure the outcome from moment to moment is a lot less clearly outlined, and in a well designed plot adventure there should be numerous results or actions which still lead you onto the next plot beat.
As for Locations, Plot tends to not care about the fictional space in which the narrative is set in, beyond the obvious effects it will have on the actual plot. Abandoning the plot that supposedly everyone has agreed to follow through on as part of the gaming session to simply explore some other part of the location is often seen as disruptive behavior in a Plot-focused adventure, whereas it is the entire point of gaming in a Location adventure.
Obvious Example: A mysterious crime has taken place, and the players take on the roles of investigators trying to uncover what has occurred, and separate truth from lies. Numerous plot points involving questioning suspects, gathering clues and uncovering layers of mysteries eventually lead to a logical conclusion of the plot in which the investigating players confront the perpetrator.
There are, of course, other ways of thinking about how to structure gaming material for play at the table, but to me these three are the major categories that tend to cover most experiences happening in tabletop RPGs.