The result of airodump-ng lists some AP's as both OPN and WPA2. How does the authentication work in that case?
12:34:56:78:AB:CD, 2011-09-09 15:29:47, 2022-10-21 16:09:05, 1, WPA2 OPN, CCMP, MGT, -75, DeLorean Free Wifi
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This is from the bug report for the same (https://github.com/aircrack-ng/aircrack-ng/pull/1868)
Open association is allowed (along with WPA and/or WPA2) to transfer unencrypted EAP packets between AP and client.
You can see the authentication value is MGT, indicating the authentication is handled elsewhere, not on the AP. The open part will be to send information to an authenticating device. Once authenticated, it will use WPA2.
You can read a bit more about it here: https://help.ui.com/hc/en-us/articles/115007253447-Intro-to-Networking-AAA-802-1X-EAP-RADIUS