How do you ensure compatibility with different platforms or devices?

Mostly, it's about rigorous testing on various devices and browsers. Emulators and tools like BrowserStack do help.

We started by launching on macOS, because there is less variance and because this is the platform on which we use our own product, so we automatically keep testing it there in real-world conditions. We have built QuikFlow with Flutter, so we can then port it to additional platforms. Likely we will find some testers for each new platform who will get access to pre-release versions and can find issues before we release it to the public (in addition to testing and code reviews during development).