I'm here to provide answers/information to your questions about credit memos in QuickBooks Desktop (QBDT), @Nagakambing.

 It is not automatic but there is an easy way to apply the credit memo to an invoice.

There will be a prompt when you record a credit memo. Please see the screenshot below:

A window will appear displaying the pre-selected invoices when you choose the last option. Click Done to proceed after choosing the correct option.

The credit memo is now applied to the chosen invoice.

Furthermore, I have provided this useful reference that compiles articles aimed at assisting you in organizing your customer transactions: Get started with customer transaction workflows in QuickBooks Desktop.

We’re here to offer our full support in getting you acquainted with our product and make sure you have everything you need to be successful. If there’s anything else we can do to help, please get in touch with us by commenting on this post.