Q: Loop over all tokens in the body of a macro

In response to the OP comment, "Ultimately, I want to list all macros in the definition of zz."

EDIT: I reformulated my answer with the tokcycle package because I found the original answer from 3.5 years ago did not handle the case well if control sequences were embedded down inside groups inside the macro. Now, nested groups pose no challenge to the task.

documentclass{article}
usepackage[T1]{fontenc}
usepackage{tokcycle}
newcommandlooptokens[1]{expandafterlooptokensauxexpandafter{#1}{#1}}
newcommandlooptokensaux[2]{%
  The macro detokenize{#2}contains these control sequences:
  tokcycle{}{processtoks{##1}}{detokenize{##1}}{}{#1}}
begin{document}
defzz{What textit{textsc{macros like today}} are contained 
  textbf{in this argument} relax??}
looptokens{zz}
end{document}

Poor man's way. If I understand correctly you want all macros encountered in the meaning (even within braces).

documentclass{article}

makeatletter
deffetchsubmacros#1{deffsm@temp{#1}expandafterfsm@imeaningfsm@temp~}
catcode`& 7
deffsm@i#1->#2~%
   {begingroupttfamilycatcode`{=9 catcode`}=9 everyeof{&}%
    makeatletterscantokens{fsm@ii#2}}
deffsm@ii#1{ifx&#1endgroupexpandafter@gobbletwofifsm@iii#1}
catcode`& 4
deffsm@iii#1{ifcatrelaxnoexpand#1Found: string#1parfifsm@ii}
makeatother                       

begin{document}


fetchsubmacros{What textit{foo macros} are bar contained textbf{in emph{this argument}} relax??}

end{document}