Is an IPad pro compatible with an IPhone XS Max please?

iPhones and iPads don’t “synchronise” with each other. They can be logged into the same iCloud account and share information across iCloud. You should have no problem doing this with a new iPad and your existing phone.

Additionally, nothing requires the respective Operating Systems of an iPhone (iOS) and iPad (iPadOS) to be on the same “version number.”

Apple has actually released those OS updates on different calendar schedules in the past.

That said, your Xs Max is capable of being updated to the latest OS (iOS 17) when it is released this Fall.

iOS 17 - Apple