this has nothing to do with quoting

  also='another way to pass variables to child scopes' sh -c "echo $also"

Neither of those are passing variables, those are setting environment variables. Bash (probably most shells?) mixes its variables with environment variables, and "export" is how you promote a bash variable to an environment variable. The inline version (GGP's [THE=tell the] version) sets the environment variable only for that one executable, while an exported one persists.

Also I'm guessing you didn't test that, you got the quoting wrong (it prints nothing, the second one should be single quotes) ;)

They are both variables. I just wanted to show how it's different from export

no I'm on my mobile so I sure did not, should be sh -c 'echo "$var"'