What is the file (or files) that allow me to access all the variables in Smarty templates written in other PHP files?
I can't find a function to do that.
there's not a file where you find all the smarty variables. To get one (or more) variables instantiated from other files you have to call a smarty method.
For example, if you have developed a module you have to call:
$myvar = $this->context->smarty->GetTemplateVars('myvar');