I have one series with roughly 2.5k chapters that are about 1k words per chapter and it's not finished yet (and has additional extras and prequels that I haven't even counted in this). There are one and a half volumes still left and I'll add some parts in the revision so I assume this will easily hit 3k chapters. I guess that's roughly the equivalent of 1k chapters with 3k words. Another series of mine is at 500 chapters with 1k words and will likely hit the 1k chapters as well although I don't think it'll go too far past that. Based on the experience of writing these two, I think there are several points that help with making a story long while not being repetitive:
1) Have a large cast while writing multiple PoVs or using an omniscient PoV. The characters should experience some parts of the story together but should also have separate experiences. Detailing all of this can easily double (or more) the number of chapters needed compared to those that only follow one MC. Having characters on opposite sides that only ever meet in conflict is likely to add the most chapters but even those working together can work as long as you split them up often and long enough.
2) Have the story span a long period of time. This can most easily be done by having immortal or near-immortal characters. Mortal characters are alright though but in that case, it helps to start with their childhood. Fill in parts about the past. For immortals, that can be their own. For mortal characters, go back one or two generations or even several if it fits the story. This might not add as much content as a larger cast does but it adds depth to the characters as it makes up their backstory and usually highlights their motivation, goals, and similar. If the past contains several of the characters from the cast or characters related to them, this part can often be made longer as it won't feel redundant to readers.
3) Set the story in a big enough place that you start to set up from the very beginning. This can mean several cities, neighboring kingdoms, different realms, or whatever fits your story. Make these places detailed and distinct. This comes not only with physical descriptions but also with cultural ones. For example, if you go with kingdoms or realms, they could have different festivals that might be similar or very different depending on what you want and can have an influence on the story. Including these details makes the world feel more lived-in and can give a better background for the characters while potentially adding dozens of chapters. Different places also have the chance to add many supporting characters and some mini-arcs here and there. Establishing a general idea of this at the beginning is important to ensure it won't feel like you're pulling all of this out of your ass. If you have multiple leading characters, this is easy to do by putting them in different places at the beginning. Otherwise, it can also work well if a lone MC has the goal to go to another place or has a vendetta against somewhere there or anything else that relates them to other places.
4) Add "mystery" in the sense that you don't let your characters know everything at the beginning or at once when it becomes relevant. Have them work to get the knowledge they need. For example, if there's a conspiracy, don't have them be already acquainted with those in the know but make them slowly chase clues. Throw in some confusing parts that they'll have trouble figuring out. Proceed with caution though: Many readers want an instant payoff and will be incredibly turned off by having this kind of mystery in the story.
5) Allow your characters to experience setbacks. I believe this isn't (or at least shouldn't be) unique to long stories but it sure helps with adding chapters: Make your characters lose and sometimes even lose badly. This way, they might need to start over what they were doing before. If, let's say, the investigation from the previous example led to a dead-end, they'll have to go back to the beginning and re-examine what they thought they knew, chase new clues, and then come to a new conclusion. Don't overdo it (as in, make it happen more than once with the same thing), instead, make it a learning opportunity for the characters that you can even reference down the line and show how they learned from it and are now doing better.
Sometimes, setbacks could also mean that their plans get ruined and they need to find alternative ways. E.g., imagine your character needs the prize from a competition to do some magic voodoo thing but they only make second place and the first place isn't willing to trade. They'll need to come up with an alternative solution to their problem now. They might need to brainstorm ideas, then implement them which could require a whole ass mini-arc (imagine they find something the first-place winner is willing to trade for after all but it's that dangerous to get kind of thing or is in the possession of another person, whatever you can think of), and only then, they actually manage to do what could have ended with the competition.
Setbacks can either come from outside or even from the character, depending on whether you make them OP or not (not doing so is usually favorable to the character count as well). If you're not sure where to put these, just always ask yourself "Well, what could go wrong?", then make exactly that happen.
Overall, I think those would be my first go-to elements to allow the story to get bigger. I believe these can be used in almost any kind of setting and, if done well, still won't feel repetitive. Instead, they can even add to the story because they give more insight than a shorter story would. Much of this relies on very distinctive characters and places though so those parts should be planned out well or at least kept in mind when adding later.