How to connect to unsecured Wi-Fi networks on MacBook Air M1 chip

The usual reason is that it is a "captive network" for which a login screen should automatically appear. Sometimes that does not happen. Try this: in Safari, type the following URL

captive.apple.com

Sometimes that will force the "login screen" to load.

The "unsecured network" status message itself is merely informative. In the case of "captive networks" its only useful purpose is to let you know that your Mac has not authenticated with that network with a "login screen" or something that demands you check a box consenting to some arbitrary terms and conditions no one reads anyway. Until you do that the publicly available router's open network will refuse to connect you to the Internet.

If what I wrote does not load that login screen then try loading a commonly used website like www.apple.com for example. Try others also. Try toggling Wi-Fi off and on again.

As before... sometimes that will work too.

This Apple Support document explains how it works on iOS devices; unfortunately macOS has yet to catch up to doing things the same way: Use captive Wi-Fi networks on your iPhone or iPad - Apple Support

If none of John's excellent suggestions work, try typing in an ip address in the browser address bar. This sometime forces open the login window.