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I was looking for more fonts like mathcal, mathbb to use them in math mode, so I can distinguish different quantities, and I found this answer on TeX.SE to the question

What are all the font styles I can use in math mode?

In the first answer there is a list of all usable fonts in math mode which can be found at the end of the mathalfa documentation and it's like

Now, I read the poor documentation on the mathalfa package, without understanding enough to get those fonts. My question is: how do I use those fonts in math mode? Which package do I need to use them?
What are the commands with which I can choose between fonts?

Basically what I would like is something like (just to make an example)

Text [...] $esstix{A} = euler{A}$ 

What do I have to do? Even a more complete documentation with some examples would be good...

mathalfa allows you to map fonts to the four standard command mathcal, mathscr, mathfrak, mathbb:

documentclass{report}

usepackage[cal=esstix,frak=euler,scr=boondox,bb= pazo]{mathalfa}

begin{document}

$mathcal{Esstix ABC}=mathfrak{Euler ABC}= mathscr{Boondox ABC}=mathbb{Pazo ABC}$

end{document}

I'll post my own answer as I found a pretty simple way to achieve the resut I was looking for.
With the command DeclareMathAlphabet, which syntax is

DeclareMathAlphabet{<math-alph>}{<encoding>}{<family>}{<series>}{<shape>}

you can define <math-alph> to be a new math alphabet.

The arguments <encoding>, <family>, <series>, <shape> are the default
values for this math alphabet in all math versions; these can be reset
later for a particular math version by a SetMathAlphabet command. If
is empty then the is declared to be invalid in all
versions, unless it is set by a later SetMathAlphabet command.

Checks that <math-alph> can be used and that <encoding> is a valid
encoding scheme.

In these examples, foo is defined everywhere but baz, by default, is
defined nowhere.

DeclareMathAlphabet{foo}{OT1}{cmtt}{m}{n}

DeclareMathAlphabet{baz}{OT1}{}{}{}

(from Index of /ctan-doc/macros/latex/doc/html/fntguide)

All the combination available with the mathalfa package are available in the source code of mathalfa.sty on GitHub. Another list, with all the font abbreviation can be found on the mathalfa documentation (at the end of the pdf).

I found particularly illuminating the answer to this question provided by the user cfr, which I thank for the illustration of the use of command
DeclareMathAlphabet.

I'll provide a simple example for future visitors:

%PREAMBLE

DeclareMathAlphabetmathzapf{T1}{pzc}{mb}{it}

DeclareMathAlphabet{mathchorus}{OT1}{cmtt}{m}{n}

DeclareMathAlphabetmathrsfso{U}{rsfso}{m}{n}

% ...



%DOCUMENT

[

    mathzapf{ABCDEFGHIJKLMNOPQRSTUVWXYZ}

]

[

    mathchorus{ABCDEFGHIJKLMNOPQRSTUVWXYZ}

]

[ 

    mathrsfso{ABCDEFGHIJKLMNOPQRSTUVWXYZ}

]

which output is