The app can be created from any service that has its desktop version.

There are two types of Sidekick apps:

⇛ a default app from the catalog

⇛ a custom app made from the URL or the opened tab (the Pro plan feature)

You can add apps both ways, using the + icon on the sidebar or the new tab page:

Choose any apps from the Sidekick catalog or create the app from any opened tab or specific URL:
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To remove any app from the sidebar, drag and drop it from the sidebar:

If you want to delete any app in general, open a new tab page (your Workspace) and follow these steps: