There are two common ways for one system to learn that something changed in another. Knowing the difference helps you choose a design that is fast and cheap to run.
Polling
One system asks the other every few minutes whether anything is new. It is simple and works almost anywhere, but it wastes effort and can be slow to notice a change.
Webhooks
The other system sends a message the moment something happens, such as a payment or an order. It is quicker and lighter, but you must be ready to receive messages and handle ones that arrive twice or late.
Which to choose
Use webhooks when the other system offers them and speed matters. Use polling when it does not, or as a safety net that catches anything a webhook missed.