How can I align arrows in different schemes?

Question

\documentclass[margin={2mm 2mm}]{standalone}
\usepackage{chemfig}
\usepackage{amsmath}

\newenvironment{aasa}{$\displaystyle\aligned}{\endaligned$}

\begin{document}

\begin{figure}[h]

\centering
\scalebox{.7}{
    \begin{aasa}
    \setchemfig{scheme debug=false}
    \textbf{Step 1} \\  
    &\schemestart
            
    %%%%%%%%%%%% STEP 1%%%%%%%%%%%%%%%%%%%%%% 
    \chemfig{
        -[:30]
        -[:-30]OH
    } 
    \arrow{->}
    \chemfig{
        -[:30]
        =[:-30]O 
    } +
    \chemfig{
        H_{2} 
    } 
    \schemestop
    \\
    \textbf{Step 2a} \\  
    &\schemestart
            
    %%%%%%%%%%%% STEP 2a%%%%%%%%%%%%%%%%%%%%%% 
    \chemfig{
        -[:30]
        -[:-30]OH
    } 
    +
    \chemfig{
        -[:30]
        =[:-30]O 
    }
    \arrow{->}
    \chemfig{
        -[:30]
        =[:-30]
        -[:30]
        -[:-30]OH
    } +
    \chemfig{
        H$_{2}$O
    }
    \schemestop
    \\
    \textbf{Step 2b$_{1}$} \\  
    &\schemestart
            
    %%%%%%%%%%%% STEP 2b1%%%%%%%%%%%%%%%%%%%%%% 
    \chemfig{
        -[:30]
        -[:-30]OH
    } 
    +
    \chemfig{
        -[:30]
        =[:-30]O 
    }
    \arrow{->}
    \chemfig{
        HO-[:30]
        -[:-30]
        -[:30]
        -[:-30]
        -[:30]OH
    } 
    \schemestop
    \\
    \textbf{Step 2b$_{2}$} \\  
    &\schemestart
            
    %%%%%%%%%%%% STEP 2b2%%%%%%%%%%%%%%%%%%%%%% 
    \chemfig{
        HO-[:30]
        -[:-30]
        -[:30]
        -[:-30]
        -[:30]OH
    } 
    \arrow{->}
    \chemfig{
        -[:30]
        =[:-30]
        -[:30]
        -[:-30]OH
    } +
    \chemfig{
        H$_{2}$O
    }
    \schemestop
    \\
    \textbf{Step 3} \\  
    &\schemestart
            
    %%%%%%%%%%%% STEP 3%%%%%%%%%%%%%%%%%%%%%% 
    \chemfig{
        -[:30]
        =[:-30]
        -[:30]
        -[:-30]OH
    } +
    \chemfig{
        -[:30]
        -[:-30]OH
    } 
    \arrow{->}
    \chemfig{
        -[:30]
        -[:-30]
        -[:30]
        -[:-30]OH
    } +
    \chemfig{
        -[:30]
        =[:-30]O 
    } +
    \chemfig{
        H$_{2}$O
    } +
    \chemfig{
        H$_{2}$
    }
    \schemestop
    \\
    \end{aasa}
}

\caption{}

\label{}
\end{figure}

\end{document}

Answer 1

the command used to do the alignment does not work within the "\schestart \schemestop" environment. It is necessary to close this environment to place the "&" alignment

You don't need $ ...$ inside \chemfig to make \chemfig{H_2O}

\documentclass[border=2mm]{standalone}
%\documentclass{article}
\usepackage{chemfig}
\usepackage{amsmath}



\newenvironment{aasa}{$\displaystyle\aligned}{\endaligned$}

\begin{document}
    

\begin{aasa}

                \textbf{Step 1}  & &  \\ 
%               %
                & \schemestart \chemfig{-[:30]-[:-30]OH} \schemestop & &\schemestart \arrow{->} \chemfig{-[:30]=[:-30]O} \+ \chemfig{H_{2}} \schemestop  \\
                %
                \textbf{Step 2a}  & &\\ 
                % 
                & \schemestart \chemfig{-[:30]-[:-30]OH} \+ \chemfig{-[:30]=[:-30]O} \schemestop & & \schemestart
                \arrow{->} \chemfig{-[:30]=[:-30]-[:30]-[:-30]OH} \+ \chemfig{H_{2}O} \schemestop \\
                %
                \textbf{Step 2b$_{1}$}  & &\\
%               %
                & \schemestart \chemfig{-[:30]-[:-30]OH} \+ \chemfig{-[:30]=[:-30]O} \schemestop & & \schemestart \arrow{->} \chemfig{HO-[:30]-[:-30]-[:30]-[:-30]-[:30]OH} \schemestop \\
                %
                \textbf{Step 2b$_{2}$} & &\\
                %
                 & \schemestart \chemfig{HO-[:30]-[:-30]-[:30]-[:-30]-[:30]OH} \schemestop & & \schemestart \arrow{->} \chemfig{-[:30]=[:-30]-[:30]-[:-30]OH} \+ \chemfig{H_{2}O} \schemestop \\
                %
                \textbf{Step 3} & & \\  
                %
                & \schemestart \chemfig{-[:30]=[:-30]-[:30]-[:-30]OH} \+ \chemfig{-[:30]-[:-30]OH}  \schemestop & &\schemestart\arrow{->} \chemfig{-[:30]-[:-30]-[:30]-[:-30]OH} \+ \chemfig{-[:30]=[:-30]O} \+ \chemfig{H_{2}O} \+ \chemfig{H_{2}} \schemestop 
            \end{aasa}

\end{document}