You could set up a LAN-only WiFi hotspot that doesn’t connect to the Internet if you have the right tools. For the most part, though, you’ll just need to disconnect the Internet.
You can set your phone to forget all other Wi-Fi networks individually or by resetting network settings, and it will automatically connect to known networks.
Once you’ve successfully joined to this non-Internet Wi-Fi network, it will rejoin automatically in the future, without requiring you to connect to any other Wi-Fi networks.
Also, turn off Wi-Fi Assist to prevent your phone from using cellular data to augment a bad Wi-Fi connection.
Yes, you can connect to a WiFi network without internet for local tasks like file sharing, printer access, or using a local server. This is useful in various offline network setups.