The description for the Emblem Holy Symbol is as follows (PHB, p. 151):
Holy Symbol. A holy symbol is a representation of a god or pantheon. It might an amulet depicting a symbol representing a deity, the same symbol carefully engraved or inlaid as an emblem on a shield, or a tiny box holding a fragment of a sacred relic. Appendix B lists the symbols commonly associated with many gods in the multiverse.
It weighs nothing and costs 5gp. The fact that it weighs nothing makes sense because it isn’t really an object that can exist by itself; it is an engraving or inlay on a shield.
So my question is, if you walk into a shop and “buy” an emblem (it costs 5gp, so it must be possible to purchase it), what exactly are you buying?
Are you buying some kind of “official pattern” that is required for a shield to count as having a proper “holy symbol” (as in, satisfies the material components for spellcasting) emblem on it? Almost like a “patented pattern”? Then why couldn’t you just buy one and replicate it yourself for free forevermore after that initial purchase?
Are you buying the “manual labour” to actually engrave/inlay the thing onto your shield? Then would it make sense that this can only be purchased from a blacksmith rather than from the local temple (where one might expect the other holy symbols, amulet or reliquary, to be purchased)? And could someone with Smith’s Tools (and proficiency with them) not do it themselves for free?
Are you effectively buying a shield that has already been made with that engraving/inlay “officially done”, so you’re basically buying a shield for 15gp (or a 5gp markup on whatever shield you’re buying), and you can’t actually buy just the emblem by itself for 5gp?
What I’m really getting at here, if I use my real example, is that in a game I am DMing, the party’s paladin (who has a shield with an emblem on it as per starting equipment) has raised concerns about having to buy a new holy symbol if they get a +1 shield in the future. This is unlike any other holy symbol, which you simply have forever (assuming you don’t lose or destroy it), since it is it’s own distinct object, so the emblem is a unique case here.
Just following the rules of what is required for a shield to satisfy the material components for spellcasting, I would say that they could just “buy” a new emblem and apply it to their new shield. But that got me thinking, what exactly would the paladin be buying at that point, and would it be possible to simply transfer the symbol over from their old shield to the new shield at no cost? This is a unique case, as for the other holy symbols, replacing some other piece of equipment would not impact them like this.
So, what exactly is an Emblem with regards to what a PC would be buying in-universe?
Note that I’m not looking for opinions, and if the answer is simply “this is not explained anywhere, as DM you have to decide what this means”, then that’s the answer.