how use ajax to custom page template

Every AJAX requests pass through the admin-ajax.
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How to use AJAX precisely in WordPress Custom Themes?

1. Step 1: Create a template file and put below code into your file. ...
2. Step 2: Localize your script into functions.php.
3. Step 3: Add function that load posts into functions. ...
4. Step 4: Place your AJAX JS into ajax.