I've found for myself that thinking of Chakra as something like a "flame" has worked out pretty well.
Not in the sense that it's literally just fire, but in a metaphorical way that it has the properties of a combustion-like "reaction" between the two energies of body and mind within a person:
> A fire needs 3 things to burn: 1. some flammable substance (fuel), 2. oxidizer, 3. a source of heat/ignition to start the flame.
If one of those is no longer supplied, the fire goes out.
Now with Chakra, what acts as fuel and oxygen for the flame are physical and mental energy inside a person.
Again, not like these energies are chemically the same as fuel, but they fulfill the same role in the reaction that creates Chakra.
A flame will continue to burn as long as it can feed on enough fuel and oxygen to sustain the reaction.
A shinobi will have Chakra as long as they have physical and mental stamina they can supply into their Chakra reaction to keep it going.
> But you can have all the fuel and oxygen you want, without a spark/ignition there won't be a flame.
That 'spark' is what Hagoromo gave people when he "gave them Chakra", and it is what is passed down by generations of shinobi: the ability to ignite your energies into Chakra. People always had the base materials for creating Chakra (phys. & mental E), but alone these two wouldn't just become Chakra without an outside ignition
> This is why Chakra can be divided without diminishing: You can light another candle with an already lit candle, and it'll produce a new source of fire/Chakra.
A person will not only posses but also (re-)generate Chakra through having some pre-existing Chakra shared with them because it is not some substance injected into them but rather a new additional component to their innate metabolism /energy circulation which gets "activated" and which you can learn to adapt to and that will then enable you to do many kinds of stuff normal people without that "extra circuit" can't.
> Natural energy just is a different or additional type of fuel for the Chakra reaction (as it is a different type of energy besides physical & mental), it can make your Chakra burn more intense/increase the volume of flame when the right amount is added to the mix, but too much can make your body/mind completely "burn out" (turn to stone)
> The Chakra fruit contains atleast one of the 2 following things:
1. huge amounts of natural energy that can act as fuel for the Chakra of anyone eating it
2. the "catalyst" that can start the reaction between physical & mental energy to create Chakra in the first place, making it a "Chakra fruit" not because it contains Chakra, but because it can awaken Chakra within anyone who eats it.
I'd say it's probably both.
Of course this metaphor has its limits but I think it is a nice way of seeing Chakra: Not as any kind of substance or energy, but as a process.
Also, this makes Hagoromo basically a Prometheus archetype, stealing flame (chakra) from the Gods (Kaguya) to share it with humans, but in doing so he incurs divine wrath upon himself