I'm not sure how to use nested for loops in bash. How do I turn the following into a nested for loop?

~/
❯ cat sequential.sh
a=(1 2)
b=(3 4)
c=(5 6)

for e in "${a[@]}"
do
    echo $e
done

for e in "${b[@]}"
do
    echo $e
done

for e in "${c[@]}"
do
    echo $e
done

✦ ~/
❯ bash sequential.sh
1
2
3
4
5
6

I'm not sure how to use a nested for loop in bash. How do I turn the below code into a nested for loop?

~/
❯ cat sequential.sh
a=(1 2)
b=(3 4)
c=(5 6)

for e in "${a[@]}"
do
    echo $e
done

for e in "${b[@]}"
do
    echo $e
done

for e in "${c[@]}"
do
    echo $e
done

✦ ~/
❯ bash sequential.sh
1
2
3
4
5
6