How to include a PHP library

Dear WordPress comunity,

I am having trouble including a php library. Mainly because of lack of knowledge.

Basically i have a php library file >> example.inc.php
-I uploaded it to the root of the website folder

Now i have to include this code onto every page and post:
-<?php require_once(“example.inc.php”); ?>

Question: Do I put this above code in the header.php (seems logical because the header is loaded on every page, but maybe totally NOT done)

Then I need to add another code on each and every page and post:

<?php
if (isExampleYes()) {
echo “Example is Yes”;
}
if (!isExampleNo()) {
echo “Example is No”;
}
?>

My best guess is again to put it in the header.php… but maybe it just isn’t that simple.

Your thought on this matter are very much appreciated.

Best Regards,