I have an app that works offline only and doesn't need a web server. I want to create an app clip for this app but most tutorials out there say I need to add a domain to my web server.
I want to share this app clip URL to my users using Messages.app. Can someone guide me to a working solution?
You need to have a web site so that you can create an association between your domain and the App Clip/App. See Associating your App Clip with your website.
The URL you use to invoke an App Clip is a URL from your web site (say https://my.domain.com/MyGreatApp - iOS uses the domain association to link this URL to your App Clip and App. When the user accesses this URL from their device it knows to either offer the App Clip, launch the App Clip (if the user already has it) or launch the full app (If the user has installed it).
Without a domain and a website there is no way you can provide a URL to launch your App Clip.
If you add a smart banner to your web site then the user also gets an improved experience in the Messages app; They see an App Clip card that shows information about your app rather than a simple text link.