This is a very popular question within metaphysics and I'd like to understand what do we are really mean with the question.
In particular, what do we mean when we use the words "fundamental" and "nature" when posing this question?
This is a very popular question within metaphysics and I'd like to understand what do we are really mean with the question.
In particular, what do we mean when we use the words "fundamental" and "nature" when posing this question?
Depends on whom you ask
In short, this is a meaningless question!
In short matter is reality (and it doesn't matter that no one quite knows what's matter)
In short reality is about meaning and meaningfulness
In short:
All consciousness is consciousness of something — Husserl
The body is our general medium for having a world — Merleau Ponty
In short the ultimate reality, whether it be left nameless or called Tao, Brahman, Pleroma or simply God is the only reality. What we usually call reality is just shadows. Vide. Plato's cave
As mentioned in some of the other answers, the response to the question "What is the fundamental nature of reality?" and the meaning attached to the terms used in that question, vary greatly with the speaker. One interesting case is Leibniz. Today we generally understand such a question as referring to physical reality taken as the fundamental point of reference. The fact that such an interpretation is dominant today is due to a victory of certain viewpoints that have their origin in 17th century European thought with its emphasis on rationalism. Yet in the 17th century, Leibniz had a radically different view of this. In his mature thought, there is a clear separation between the physical realm (which he described as the realm of the phenomena such as the rainbow) and the fundamental (and ultimately the only real) realm of minds/souls/monads (in different periods he would use different terms). Leibniz also clearly realized (unlike some of his enthusiastic followers such as l'Hospital) that the realm of mathematical abstractions is separate from the physical realm, and described some of the mathematical entities he worked with as "useful fictions" (negatives, imaginaries, infinitesimals). To Leibniz, the answer to the question about the fundamental nature of reality would be to explain that ultimate reality resides in the monads, whereas physical reality is merely a phenomenon.
The nature of the Leibnizian monad is not easy to account for exactly, especially from the modern viewpoint (as mentioned above). But in first approximation, it is an indivisible unit of the metaphysical world, a kind of metaphysical atom. For this reason, some mathematicians have been tempted to compare it to indivisibles, atoms, or infinitesimals, but to Leibniz such comparisons would involve a "category error": physical atoms (whose existence Leibniz denied) belong to the phenomenal realm, whereas monads belong to the only true metaphysical realm.
When I ask it, I'm usually asking something along the lines of "Why does it work the way that it works? And in what ways, exactly, does it "work"?"
I would say the fundamental nature of reality is existence.
it is held that 'Being' is of all concepts the one that is self-evident. Whenever one cognizes anything or makes an assertion, whenever one comports oneself towards entities, even towards oneself,1 some use is made of 'Being'; and this expression is held to be intelligible 'without further ado', just as everyone understands "The sky is blue', 'I am merry', and the like. ... The very fact that we already live in an understanding of Being and that the meaning of Being is still veiled in darkness proves that it is necessary in principle to raise this question again. (Heidegger, Being & Time, H. 4.)
Probably my downvoter thinks 'existence' is just a word and that we should be thinking of proto-matter as fundamental, but in the sciences the observer cannot be written-out, try as they do.
Sciences are ways of Being in which Dasein comports itself towards entities which it need not be itself. ... [However] whenever an ontology takes for its theme entities whose character of Being is other than that of Dasein, it has its own foundation and motivation in Dasein's own ontical structure, in which a pre-ontological understanding of Being is comprised as a definite characteristic.
Therefore fundamental ontology, from which alone all other ontologies can take their rise, must be sought in the existential analytic of Dasein. (ibid., H. 13.)
Today this questions asks for a principle from which derive explanations of the main phenomena in our world.
Originally, e.g. at the time of the Greek philosophers of nature, the question asks for a primordial substance. One strand of answers pointed to the elements like water, fire, earth, aether. Later the Pythgorean schools added the principle of coding with numbers or elementary number theory. Theists attempt to derive their explanantion from a personal being like a god. Candidates from physics are a unified theory like quantum gravity or a unified field theory.
If one asks for a single principle suitable as a heuristic to explain many phenomena on quite different fields, I consider “evolution by self-organization” a good candidate.
But I do not see any candidate which could serve in the all-embracing sense of your question.
I can imagine the only possible answer you're looking for here is
"What are the common and/or popular opinions, on this matter, ideally from academics or others who are leaders or at least well-known in the field?"
It seems weird and unacademic to reference a "youtube" series, but (surprisingly) if you click to this ex-neuro-scientist's page,
https://www.youtube.com/@CloserToTruthTV
There are extensive interviews with what can only be described as every single leading figure in the relevant fields, answering in their view precisely the question you ask.
I say "every" carefully because I have never found anyone not on there - whether wankers like Bostrom, major figures like Dawkins, PR creations like Kaku, obscuritanists like Chaitin, Raman, etc ... endless list ...
(The interviews also cover other unanswerable dribble like "Fermi!! Paradox!!", "Is! Google! Conscious?!!?", etc.)
For the record
I absolutely despise, loathe, the internet both in principle and practice, and in particular tiktok and similar sites; I do not recommend anyone uses the internet. But it's difficult to argue with this mind-boggling source of source material on precisely the question you ask.
For example, if you look to the excellent review in the @Rushi answer above, for those six categories you can instantly find, the current exposition, of the most famous "leading" academic, in precisely each of those categories, instantly, on that one weird youtube channel.
(I've realized you can literally put "fundamental" in the URL, good grief! https://www.youtube.com/@CloserToTruthTV/search?query=fundamental )
Just two random examples, you have both Susskind and Penrose on there giving their exact current answer to exactly your question (again, no real reason to pick those two examples, since "everyone" of any note at all is on there); and I'm not diminishing the thoughts of Random Interneters found on an S.E. site, but, I assume you're looking for "current thinking from academics on the matter" when you use the royal or species-esque "we" - hope it helps!