Disclaimer, the following is more from my Mahayana related readings integrating to my understanding of emptiness. But I am pretty sure there’s no problem for anyone to accept the following arguments.
Let’s say there’s a wooden thing, with a flat rectangular surface, and four cylindrical things near each end of the surface, on the same one side.
I can just label it as table. Which is the convention of things in that general shape and size used for the function of eating on top of it, or putting computers on top of it, or books etc. Or I can label it as chair, so people in this alternate world has no concept of table, they don’t eat on the object above, but sit on it, and eat holding the bowls by hand. They don’t put computers on it, they put laptops on their lap, sitting on the chair (which looks like a table to us) and work on their laptops.
By convention, there’s a sort of a concept laying on top of something. Based on the concept, the thing can function. If I label the thing as trash, then it’s basically useless to me. I can also label it as fuel for fire, so it gets burned.
Another example is cryptocurrency. It’s just 1 and 0 and lots of codes, communication with computers in a certain manner, but some people have this idea of putting a label on it as bitcoin etc, to function as a store of value.
Money as the concept itself is also mere conventions. It’s up to the collective humanity to designate what’s to be considered as a store of value. So knowing that it’s mere convention, one doesn’t need to be dogmatic and say only paper money is real money, or only gold is real money. We are already used to digital money in terms of online banking, credit cards etc way before cryptocurrency made it to the scene.
Does money truly exist on its own? Does table, chair truly exist on their own? Or does it depends on minds designating them to exist? If something doesn’t exist independently, they are empty of inherent essence, thus empty of self. Mere conventions.
Another example: companies, corporations, like Apple, Google, etc. Do they truly exist? Can point to me which building is Apple? Which product can capture the true essence of Apple? Or is it actually a collection of stuffs we conventionally label with as Apple? Does Apple truly exist on it’s own?
Apply all these to the person/self, being etc.
Note that no self doesn’t imply immorality, there’s still the 5 aggregates and kamma is still generated by unenlightened ones.
Are conventions only known as ‘conventions’ by the enlightened? For example, if a mother worries about her children, I doubt her children are cognizant conventions to her. Instead, her children would be real beings or sakkaya to her; thus fraught with potential dukkha.