Sorry this is a bit of a long rant. To many people here this will be a very old hut but I've only recently realized that Apple and Google are basically extortion rackets and have made the online world way worse and enshittified than it should be. Why? They deliberately make it as hard as possible to sell true cross-platform apps. "Buy one license, install our software on all of your devices." That should sound reasonable, yet it's impossible as a business model. If you put an app on the Apple Store or on Google Play, you cannot have it unlocked by a license code. So you have to sell it on their platform. That means that if you want to sell one license for all platforms, you'd have to force your users to buy the license in one of those stores, then generate desktop license from that store purchase. That's not just nonsense, it still wouldn't work because the user can still not switch from iOS to Android and vice versa.

Okay, so sell mobile platforms separately but make desktop platforms with one license. That would still be nice and comparatively hassle-free for your customers. Not so fast, though! Desktop app stores also prohibit free apps with unlock mechanism. So you have to put unlockable versions on your website. However, app stores explicitly prohibit even just mentioning that the user doesn't have to buy an app if they have already bought it elsewhere. They strictly prohibit pointing the user elsewhere to check whether they could use it for free. Consequently, unless your users are extremely well educated about those platform quirks, many of them will erroneously buy your app twice.

But wait! There is an exception in Apple Store and Google Play Store. You're strictly prohibited from putting an app on there that needs to be unlocked by a license key, but you are allowed to ask the user to create an account or log into an existing account with a user name and email address. As long as users can also purchase the app on the respective store, that's allowed. So you can tie the app to an account and unlock it if the user already bought it for another platform. For some reason, unlocking an app with a code is strictly prohibited but unlocking it by providing your email and a password and then contacting a server is totally fine. That's why so many apps have those shitty login requirements. I wasn't really aware of that.

In a nutshell, apart from "freemium models" there are two major ways to sell cross-platform software that really is cross-platform. First, make the user pay individually for each platform, make them buy your app again and again with all the hassle that brings. Second, force them to login with an email and a user name with some direct contact to some centralized hub server.

IMHO, all of these options are strictly worse than just allowing your users to buy the app anywhere they like and unlock it for all other platforms.