in a batch file how do i retrieve an environment variable from string?

To get the value of a value of a variable, the “usual” way is to use delayed expansion.
But you can also force a second parsing (“expand an expanded variable”). You can do that with the call command (which executes the following command within a second separate process). To pass a literal % to the second process, escape it with another %, which explains the strange triple%.

@echo off
setlocal enabledelayedexpansion

set "string=varname"
set "varname=some text"

echo with delayed expansion:    %string% = !%string%!
call echo without delayed expansion: %string% = %%%string%%%