this was sparked by some letters not being able to be converted
I guess what I'm asking [if there is even a way to do this, I'm quite a novice on the topic] is if you could generate letters using ABCD, for example, and then put them on a different font, assigned to different characters?

TL;DR: Is it possible to:
1> Generate, for example 𝗭𝗬𝗫, with the glyphs in ABC
2> Take out the letters 𝗭𝗬𝗫 that have been generated with the font
3> Put them in a different font, actually assigned to the unicode of 𝗭𝗬𝗫

or really, anything akin to this - I don't know what I'm doing

You could do so with FontForge’s scripting - I used to use that to post-fix fonts coming out of BitFontMaker2. Here’s an example:

ff_uppercase.bat

@echo off
if "%1" == "" goto help
if "%2" == "" goto help
fontforge -quiet -script %~dp0ff_uppercase.pe %1 %2
goto end
:help
echo Usage: ff_uppercase in.ttf out.ttf
:end

ff_uppercase.pe

Open($argv[1]);
//
name = $1:r;
p1 = Strrstr(name, "/");
p2 = Strrstr(name, Chr(92));
if (p1 < p2)
	p1 = p2;
endif
if (p1 >= 0)
	name = Strsub(name, p1 + 1)
endif
name = name + ".uc";
SetFontNames(name, name, name);
//
n = CharCnt();
//
replaceable = Array(n);
i = 32;
while (i < n)
	replaceable[i] = 1;
	++i;
endloop
replaced = 0;
//
i = 32;
while (i < n)
	if (IsUpper(i) && WorthOutputting(i))
		Select(i);
		Copy();
		lower = ToLower(i);
		if (replaceable[lower])
			replaced++;
			replaceable[lower] = 0;
			Select(lower);
			Clear();
			Paste();
		endif
	endif
	++i;
endloop
Print("Replaced " + ToString(replaced) + " glyphs.");
Generate($argv[2]);

The Python API is probably easier to work with.