Built-in app IDs use lowercase names (macos9, hub, bluepencil); their display titles come from app.json. Older links and saved settings still work. Existing app data keeps its original storage and sync namespace.
Icons beyond the built-ins are desktop apps: folders in ~/.exe/apps, each just an app.json plus an index.html, served straight from disk — edit one and reopen its window, no rebuild. Each app gets private storage under ~/.exe/appdata plus the shared Workspace. Apps are a good thing to ask a coding agent to build for you.