Can I transfer a game purchase from one platform to another?

Short answer: you can't.

Jackbox Games does not control the various stores and marketplaces where our games are sold and as a result, you can’t transfer purchases between platforms. In order to have the game on a new platform, you must purchase the game on that platform. 

There is one exception to this rule: If you buy The Jackbox Party Pack 1, 2, 3, 4, 5, or 6 for the Apple TV, you’ll also be able to download it on the iPad, and vice versa. However at this time this does not include MacOS unfortunately!