What's the BEST way to add css &/or js to a single page withOUT adding to theme?

@Rikard, thank you!

so, just to be clear – to make sure i completely understand…

when the pages load, & the functions.php file gets read, everything inside the function with “is_page(###)” gets skipped for all pages that are not that particular page, & so avoiding any performance hits that code may cause – right?

okay, so, i followed the above link to get the “if(is_page( 3739 ))” bit. is that correct? where do i find the number associated with the pages?

next, i followed the link provided there to https://kriesi.at/documentation/enfold/add-custom-js-or-php-script and find this code (& i added the “if is page” part)…

function my_custom_function(){
if(is_page( 3739 )){
    ?>
    <script>
        // Your function here
        jQuery(window).load(function(){
            console.log('Hello World!');
        });
    </script>
    <?php
}}
add_action('wp_footer', 'my_custom_function');

…if i’m adding my code to the main Functions.php file, then why is there the “add_action(‘wp_footer’)” part there? do i change that be ‘wp_functions’ or something?

also, where in the page load does the function.php parts come in – the head or foot?
if i need to add some external script references and some js form validation and some css for the form, would that be better put in the header.php file or the functions.php? it looks like, according to the linked doc page, that the same code is used for either file.

and lastly, later in that doc page it talks about needing to use “use strict” for custom js. i’m confused by this because i’ve seen other js code added to functions like this that do not use strict mode (like even in the examples before that one). what are the criteria for needing to use this strict part?

i also noticed inconsistencies with using “jQuery” vs the dollar symbol – is that simply interchangeable?

inside a function, between the closing ?> & rebeginning of <?php, is that just an open html area? so conceivably, i could do something like this…

function my_custom_function(){
if(is_page( 3739 )){
    ?>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.19.1/jquery.validate.min.js"></script>
<script>
         $(document).ready(function() {
	     $('form').each(function() {
		$(this).validate({
                   //etc//
                });
            });
       });
</script>

<style>
form {
/*css styles*/
}
</style>

    <?php
}}
add_action('wp_head', 'my_custom_function');

…or do i have to download the cdnjs file(s) & put them in my child theme/js folder & use this example from the doc page?…

function my_custom_scripts() {
    wp_enqueue_script(
        'custom-script',
        get_stylesheet_directory_uri() . '/js/my_custom.js',
        array( 'jquery' )
    );
}
add_action( 'wp_enqueue_scripts', 'my_custom_scripts' );

…if so, then what does this “wp_enqueue_scripts” thing?

sorry, but as i got going, i realized that i’m still confused on the more detailed points.
thanks!