An htaccess rule is evaluated before WP is called to get page content. So there is no variable you could use in an htaccess rule to check if a user is logged in.
Instead, I’d add to your functions.php file code that would check the is_user_logged_in() result. If not logged in, then use wp_redirect() to redirect to a login page. The die() is important after the wp_redirect so that the redirect will work properly.
Code would look something like this (not tested):
function check_user_logged_in() {
if (! is_user_logged_in()) {
// not logged in, so redirect them
wp_redirect("https://www.example.com/login-page");
die();
}
return;}
add_filter("init", "check_user_logged_in"); // fires the function on WP init
Of course, this should be placed in your Child Theme’s functions.php file.
Added 20 Jul 2020
Corrected the code fragment to be is_user_logged_in – which I did speify correctly in the non-code portion of the answer. Thanks to @Frits for his eagle eye. (I did say it was untested code, so maybe that’s my excuse….)