Shopify plus member with access to checkout page, but how do I actually edit the code?

I’m looking to do a filter on the Pickup locations displayed based off of an external variable.

When I open the checkout.liquid file(Brooklyn theme) I don’t really see how to edit anything. I plan on adding a script to do the filtering, but I’d like to see how things are working before I begin writing the script. I’d like to see how the locations are being pulled in, displayed, etc.

I just see {{Content_for_layout}} which is where I assume the actual HTML lives, or maybe its all dynamic..

Please help me to understand what I should do, to make some changes thanks.

This will sound blunt, but said with the best of intentions. If you’re not sure how to edit the file it will be best to get someone to help that does. You really don’t want to break that file!

Many of the components of the checkout are built from those liquid drops you’ve identified. This means to alter or adapt those parts you’d need to use JavaScript to alter the rendered HTML. Other parts of the checkout (like those that generate the delivery shipping options) are also loaded dynamicaly so your code needs to wait for that to be ready before attempting any tweaks.

This is a good starting resource for the template but doesn’t deep dive into the coding requirements itself - just some high level topics.
https://shopify.dev/docs/themes/theme-templates/checkout-liquid