Steam integration, explained

The Mac Steam app only offers Windows-only games when it's told to behave like Steam on another platform, and only runs them through a compatibility tool. Turning on Steam integration (see Turning on Steam integration) therefore makes three small changes, all settings, none of them patching Steam:

a one-line settings file inside Steam's app (steam_dev.cfg) that switches Steam to its "Windows mode";
two tool links in Steam's app: Superposer (Windows version) for your Windows games and Superposer (Mac version), a passthrough that keeps every Mac game running as a Mac game;
a list in Steam's config.vdf of which game uses which tool. Windows-only games use Superposer; every installed Mac game is mapped to the passthrough, because Steam in Windows mode would otherwise treat it as unsupported.

Games you buy later are mapped before they install, so a Mac version installs the Mac build and a Windows-only game gets Superposer.

The bridge. Windows games talk to Steam through the Steamworks API. Superposer includes a small bridge, built from Valve's public Proton source, that connects the game to your running Mac Steam client. For copy protection that expects a genuine Steam client library, Valve's own Windows file comes from the free Steamworks SDK Redist, which Steam downloads for you; Superposer never redistributes Valve's files and never works around copy protection.

Steam stays up to date. Superposer changes no Steam program file, so there is nothing to "block updates" for. If a Steam reinstall removes the settings above, Superposer notices and puts them back, and says so on the Steam page. Turn Off on the Steam Integration page, or uninstalling Superposer, puts Steam back to normal; see Uninstalling Superposer.