Discover local server

On first start the app tries to discover your openHAB server. This will only work on local networks and when the server does not enforce either authentication or HTTPS. If it fails, you can click on Go to settings and manually enter the server settings.

How does application find local Openhab server? And How can I disable it?

The app uses mDNS to discover the server: Multicast DNS - Wikipedia

You cannot disable it in the app, but it only tries to discover a server if no server is configured.