How to use Awk to format numbers with a thousands separator

Can anyone tell me how to use the awk command for this statement "the number will contain commas after every three digits. For example, 100000000 will become 100,000,000." I know how to use sed command to obtain this output but I don't know how to use awk, I'm beginner please tell me I'm searching from morning but I didn't get a suitable answer for this so please suggest me best tutorials for learning. sed command to get the above output: echo "100000000" | sed -E 's/([0-9]{3})/,\1/2g'