OK, that helps: on my system

Information[ResourceFunction["SymmetricDifference"]]

yields as Function Location

LocalObject["file:///Users/myname/Library/Wolfram/Objects/Resources/\
4f3/4f3f7f00-c70a-455e-8bd4-1eaa2b455ae5/download/DefinitionData"]

...which is in a different folder than you found!

An aside: it's really annoying that while Information works on the ResourceFunction expression, neither

 ?ResourceFunction["SymmetricDifference"]`

nor

?? ResourceFunction["SymmetricDifference"]

works — each returning

  Missing["UnknownSymbol", "ResourceFunction[\"SymmetricDifference\"]"]

which seems to be contrary to what the Documentation Center page Information says in its Details section about the special input forms! This is either a bug in the version 12.0 updated Information implementation or an error or omission in the documentation for it.

Using SymmetricDifference = ResourceFunction["SymmetricDifference"] does allow one to use the function. However:

• evaluating ?SummetricDifference still gives no useful information (beyond saying it is Global`SymmetricDifference); and
• still there is no information on SymmetricDifference in the Documentation Center.

Moreover:

• Import["~/Library/Wolfram/Objects/Resources/c3f/c3fc4fd0-fb66-45c5-\ 94c2-5bce56bed0e3/download/DefinitionData/export"] gives an error that the object of the import was not found! In fact, I see no directory ~/Library/Wolfram/Objects/Resources/c3f at all.

Also:

• What is /Library/Wolfram/Objects/DeployedResources/Function/SymmetricDifference? This directory contains two files, object.wl and put.wl. How does one use that?

1. Does the Mathematica Documentation Center have a page on which that is explained? Does the Wolfram Function Repository itself? (If so, I certainly did not see it.)
2. Evaluating AppendTo[$Path, FileNameJoin[{$LocalBase, "Resources"}]] does not make such a function available. That is, evaluating, say, ?SymmetricDifference returns Missing["UnknownSymbol", "SymmetricDiference"] and an expression such as SymmetricDifference[{1, 2, 3}, {2, 3, 4, 5}] returns unevaluated — even after I have "deployed locally to this computer" from SymmetricDifference-definition.nb. Why?
3. I do know about the (experimental) function ResourceFunction. After I have "deployed locally to this computer" from SymmetricDifference-definition.nb, if I then evaluate ResourceFunction["SymmetricDifference"], I can copy/past the resulting object and use it as if it were an ordinary function. Nonetheless, ?SymmetricDifference still returns Missing["UnknownSymbol", "SymmetricDiference"] and SymmetricDifference[{1, 2, 3}, {2, 3, 4, 5}] returns unevaluated.

This all seems plain dumb!

What should one do so that such functions become available directly, as if they were built-in (or, at worst, as if they were in an add-on application that one must load)?

How does one readily see the names of all such locally deployed functions?