How to dynamically build atoms without using string interpolation or to_atom functions

Can you elaborate on why?

I’m curious as to ‘why’ as well, the only ways to make atoms is a remote message from the mesh, compiling a module with the atom, or String.to_atom or its kin as far as I recall?

Sure.
I’m correcting the documentation, which have changed for OTP 20 onwards

diff --git a/lib/elixir/lib/list.ex b/lib/elixir/lib/list.ex
index 7c2678890..a0a12ecab 100644
--- a/lib/elixir/lib/list.ex
+++ b/lib/elixir/lib/list.ex
@@ -748,15 +748,19 @@ defmodule List do
   @doc """
   Converts a charlist to an atom.
 
-  Currently Elixir does not support conversions from charlists
-  which contains Unicode codepoints greater than 0xFF.
+  As of Erlang/OTP 20, Elixir supports conversions from charlists which contains
+  any Unicode codepoint. Earlier versions did not support convertions from
+  charlists which contains Unicode codepoints greater than 0xFF.

and the “Syntax Reference”, Which I’m trying to go thoroughly through, particularly the first codepoint of an atom, without using quotes. The page states states:

Atoms

Atoms in Elixir start with a colon (:) which must be followed by a non-combining Unicode character or underscore. The atom may continue using a sequence of Unicode characters, including letters, numbers, underscore, and @. Atoms may end in ! or ?. See Unicode Syntax for a formal specification.

All operators in Elixir are also valid atoms. Valid examples are :foo, :FOO, :foo_42, :foo@bar and :++. Invalid examples are :@foo (@ is not allowed at start), :123 (numbers are not allowed at start) and :(*) (not a valid operator).

If the colon is followed by a double- or single-quote, the atom can be made of any character, such as :"++olá++".

Atoms in Elixir start with a colon ( : ) which must be followed by a non-combining Unicode character or underscore.

For instance, the non-combining charracter part is no longer valid

If the goal is to test the parser, you could generate a string of code then compile it.

What has this to do with dynamically building atoms? This looks more like quoted vs. unquoted atoms.