What do you roll to hit a patch of the ground? Not necessarily to damage it, just to hit a mark with something. An example would be a flask of oil into a space not occupied by a creature. Another would be throwing stone into a well that is a distance away.

Technically the AC is determined by the substance the surface is made of, but if the whole surface is Large (10’×10’) or larger, then that AC would count for that 10’×10’ square, to hit a specific 5’×5’ it should be higher. But that seems… needlessly dumb to me, especially if you’re not trying to actually damage the surface at all. so I would pro’ly call it AC 10 or 11 for a 10’×10’ square, and then maybe AC 13 or 14 for a 5’×5’. 💁‍♂️

I would not use attack at all. I would just ask for dexterity check. As a DM, I would define the DC based on the size of the target, distance and the thing you are throwing. Just imagine how difficult would it be in real life and define the DC as desribed in rules (DC 5 for easy task, DC 10 for medium difficulty, DC 15 for hard, etc.)

I'd say AC 10 and may impose disadvantage depending of the target's distance.

How about an AC/Dex check of just how many squares away the intended target is? It should be dead simple to drop a mark next to you or only a few feet away, but increasingly difficult to hit one farther away.

The one hole in my suggestion is that when you target the ground, it gets harder as range increases, but not when you target a creature. It would be easier to hit a rat at 140' than a square 28 grid spaces away with a longbow. Maybe you could workshop something that increased difficulty with range but still sort of fell into range of appropriate ACs. Maybe increase AC for every 10' instead? Or distinct range categories with different difficulties.

Since there is no direct rules answer, all the replies can only be suggestions.

In my opinion, the best way to handle this will depend on the particular situation that you're trying to adjudicate.  Sometimes an attack roll makes sense, sometimes an ability check makes sense.

Attack roll:

You make the attack roll. On a hit, you roll damage . . .

Some attacks cause special effects in addition to or instead of damage.

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack . . .

When you make an attack, your attack roll determines whether the attack hits or misses . . . If the total of the roll plus modifiers equals or exceeds the target's Armor Class (AC), the attack hits.

Ability Check:

An ability check tests a character's or monster's innate talent and training in an effort to overcome a challenge. The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure . . .

the DM decides . . . the difficulty of the task, represented by a Difficulty Class. The more difficult a task, the higher its DC.

For the example situations described in the OP, I would tend towards an attack roll for the oil flask (since that item description talks about making attack rolls) and towards an ability check to toss a stone into a well from a distance.

Now, to go down the rabbit hole with the oil flask for a moment, the item description says this:  "Make a ranged attack against a target creature or object, treating the oil as an improvised weapon. On a hit, the target is covered in oil."  So, technically, you cannot target a point in space or a hex / grid square on the ground.  This could be flavored a few ways when trying to throw the flask onto some portion of a stone floor.  For example, the flask doesn't shatter.  Or, it shatters but only affects a few inches of space such that it is not slippery or flammable or perhaps the resulting flame is too small to cause damage, or it occupies such a small portion of a grid square that most creatures could simply occupy some other portion of that grid square without taking damage, or whatever.

Keep in mind that the stone floor does not qualify as an object according to the DMG:

Objects:

Use common sense when determining a character's success at damaging an object . . .

 For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.

When time is a factor, you can assign an Armor Class and hit points to a destructible object. You can also give it immunities, resistances, and vulnerabilities to specific types of damage.

An object's Armor Class is a measure of how difficult it is to deal damage to the object when striking it (because the object has no chance of dodging out of the way).

Now, if you're not really trying to cause any effect that is detailed in the item description for the oil flask and you are simply trying to hit a spot on the floor for some other improvised reason, then I would probably revert back to using an ability check if there's a chance for failure instead of an attack roll.

But let's just say that instead of trying to hit a spot on the stone floor, we are trying to hit a life-sized wooden statue of a Commoner, but we are not trying to cause damage:

Note that the above rules for hitting an object with an attack roll are for the purposes of damaging and/or destroying a destructible object.  That's not the intent here, so those AC guidelines are out of whack.  It even says that the full AC value has to do with the difficulty to cause damage to a particular substance and that no portion of the suggested AC values relate to the ability to dodge since objects cannot dodge.

Logically, this causes the AC in this case to be reduced to 0.  Either this translates to an auto-success, or you would instead ask for an easy ability check to determine the accuracy of your throw if it's determined that there's some chance for failure.

Perhaps another way to model this is to say that the Commoner with Dexterity 10 would have an AC of 10, but this statue of the Commoner essentially has a Dexterity of 0 (modifier of -5) and therefore the AC for the statue is 5.  This could be flavored as there being some chance that the statue is shaped and contoured in such a way that maybe a glancing blow does not cause the flask to break, or that the texture of the statue is so smooth in some areas that the oil just slides right off of it, or whatever.

So yeah, long story short, this is one of those cases where the DM just has to determine how difficult it should be to succeed on the task and try to ask for a die roll from your player that accurately reflects the correct chance for failure, and the DM has various tools available to achieve this.