We need to write jquery sometimes in phtml file for special requirement. For that you need to define jquery in require function in script tag for using jquery within phtml file.
Put below script in phtml file and replace console.log with your custom code.

<script>
    require(['jquery', 'jquery/ui'], function($) {
        $(document).ready(function($) {
            console.log("custom jquery"); // write down your custom code from here.
        });
    });
</script>