How to Add Custom Information to Orders When Developing a Shopify Extension

[JohnnyXiao]

Hello,

I am currently developing a Shopify extension and need some assistance with adding custom information to orders. Specifically, I would like to achieve the following:

Displaying Custom Information on the Checkout Page:

How can I display custom information (e.g., customization options, special instructions) on the checkout page before the customer proceeds with the payment?
Adding Custom Information When Clicking the Pay Button:

How can I ensure that specific custom information is added to the order when the customer clicks the "Pay" button on the checkout page?
Adding Custom Information When Clicking the Add to Cart Button:

How can I add custom properties or information to the items in the cart when the customer clicks the "Add to Cart" button on the product details page?
Any code snippets, API references, or detailed steps would be highly appreciated.

Thank you for your help!