In principle they are somewhat similar: they're both Wine + local patches (either backported from more recent Wine versions or hacks that are not suitable for u

In principle they are somewhat similar: they're both Wine + local patches (either backported from more recent Wine versions or hacks that are not suitable for upstream Wine) + some glue. CrossOver also has a GUI, which Proton doesn't need because the GUI there is Steam and Proton is supposed to be as transparent as possible. And like I said for Proton, it's in our best interest to keep the diff between Wine and CrossOver as small as possible, because that's a liability that takes its toll at each rebase. In practice Proton and CrossOver tends to have rather different sets of needs. Proton only supports Linux and is only focused on gaming. CrossOver exists for a handful of platforms, most notably macOS (which has a number of intricacies: missing 32 bit libraries, Apple Silicon, missing Vulkan drivers which requires MoltenVK, but MoltenVK itself cannot really offer all the features standard Vulkan drivers have on Linux), and is also used for non-gaming applications. CrossOver also lives in the form of specialized branches we develop for specific applications, on request of their developers. So while of course we share everything that can be shared, in my experience this tends to be not that much (except what goes upstream, of course). In pratice CrossOver and Proton live in different repositories, have independent build systems and independent release schedules.