If an app can see how much memory is available, it can use this technique. All operating systems use memory when they create a new window and when the create gui widgets such as input fields and buttons.
On their own machine , the malware author monitors free memory vs used memory. The click "buy now" in the eBay app. That open a "log in to PayPal " window. The malware author notes that opening the login window caused memory usage to increase by 23752 bytes.
The malware author creates an app that monitors how much memory is used. When memory usage jumps by exactly 23752 bytes, that means the PayPal login window is probably being opened. The malicious app pops up it's own window that looks like the PayPal login window. Since the user was expecting a PayPal login window at that moment, they enter their credentials. 5. Profit!
Note there's nothing unique to any operating system here. On any systwm, an application can find out how much memory and disk space is available, and therefore infer whether or not the PayPal login window is being opened, based on the precise amount of memory that window uses as it opens.