take an example:
Synchronize: I give you a kick, you immediately tell me you hurt;
Asynchronous: I give you a kick, you don't respond. Two days later, your mother came to my mother and said you hurt.

the Synchronize here returns a QR code link, which means that when you call this API, the QR code link is saved and returned in the response result; if the payment is successful, an asynchronous message is returned: you call the payment API. The response result of the API does not contain information about whether the payment is successful. When the payment is successful, the payment platform will call the API provided by you to inform you whether the payment is successful.

digression: the dotted line return arrow in UML refers to the result return of asynchronous messages, which is not very accurate here.