A mod is a set of patches against a release of a harness like OpenCode. Install one and your opencode becomes that release with the mod built in. Your stock install is never touched, and you can switch back any time.
Every mod is a git patch series made against one release of its harness. open-mods install clones that release, applies the patches, builds it with the exact toolchain the release pins, and puts the result first on your PATH. open-mods off steps aside so the stock binary runs again.
When the harness ships a new release, CI applies and builds every mod against it. A mod that still works has its release moved forward here automatically. A mod that does not stays on its last working release, shows as behind, and its maintainers get an issue with the error and the steps to rebase.