All methods annotated with AnyThread can be called from any thread and all the asynchronous callbacks will be returned on the same thread. Methods annotated with UiThread should be called from the Ui thread and all the asynchronous callbacks will be returned on the Ui thread as well.
(https://developer.android.com/reference/com/android/billingclient/api/BillingClient)

Where i can find those annotations for methods (i didnt find any in docs)?

P.S. In Qt's "hangman" example everything runs from Qt thread (e.g. as from AnyThread), but they can miss that thing when writing example.