categories=( categoryA categoryB categoryC )

categoryA=( productA1 productA2 productA3 )
categoryB=( productB1 productB2 productB3 )
categoryC=( productC1 productC2 productC3 )

for category_name in "${categories[@]}"
do
    echo $category_name

    category=${!category_name}
    for product in "${category[@]}"
    do
        echo -e 't' $product
    done

    echo -e 'n'

done

CategoryA
    ProductA1
    ProductA2
    ProductA3

CategoryB
    ProductB1
    ProductB2
    ProductB3

CategoryC
    ProductC1
    ProductC2
    ProductC3

CategoryA
    ProductA1

CategoryB
    ProductB1

CategoryC
    ProductC1