documentclass{article}
usepackage{amsmath}

defmatOne{begin{matrix}1 & 0 \ 0 & 1end{matrix}}
defmatTwo{begin{matrix}a\bend{matrix}}
defmatThree{begin{matrix}c & dend{matrix}}

begin{document}
    begin{align*}
        begin{pmatrix}
            matOne & matTwo\
            matThree & e\
        end{pmatrix}\
        begin{pmatrix}
            1 & 0 & a\
            0 & 1 & b\
            c & d & e\
        end{pmatrix}
    end{align*}
end{document}

definemathmatrix[pmatrix][left={left(}, right={right)}, distance=1em]

defmatOne{startmatrixNC 1 NC 0 NRNC 0 NC 1NRstopmatrix}
defmatTwo{startmatrixNC aNRNC bNRstopmatrix}
defmatThree{startmatrixNC c NC dNRstopmatrix}

starttext

startformula
    startpmatrix
        NC matOne NC matTwoNR
        NC matThree NC eNR
    stoppmatrix
stopformula
startformula
    startpmatrix
        NC 1 NC 0 NC a NR
        NC 0 NC 1 NC b NR
        NC c NC d NC eNR
    stoppmatrix
stopformula

stoptext