I would like to add a pop up message after the checkout button is clicked.
Framework: shopify liquid js
The message should contain a form which allow the user input their phone number and personal ID.
The result should save to an excel (or backstage?)
You haven't mentioned if you are using a framework of any kind for your styling and features.
If you are using Bootstrap CSS then you want to look at Modals.
https://getbootstrap.com/docs/5.1/components/modal/
Note: Don't forget to change the version to the version of Bootstrap you are using.
If you are not using Bootstrap CSS and instead another framework, check the related documentation for Modal behaviour and implement as instructed.
If you are not using any frameworks or the framework you are using doesn't support modal behaviour, then please check out the links below.
Once you have built your modal, you can then add a form to the modal's body.
Good Luck!