There are several methods to discover WLCs. The methods you listed using for initial WLC discovery (not for joining). Once AP send a Discovery Request to a WLC, it will respond with "Discovery Response". That message may include other WLCs in the same mobility group. So AP can build available WLC list.
https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-5/Enterprise-Mobility-8-5-Design-Guide/Enterprise_Mobility_8-5_Deployment_Guide/cuwn.html
Once it build WLC list, it will send Join Request to one of them depend on availability configuration (primary,secondary,tertiary) .If AP does not have those config, it will look for master controller to join.
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When you are adding lightweight access points to a multiple-Cisco WLC deployment network, it is convenient to have all lightweight access points associate with one master Cisco WLC on the same subnet. That way, you do not have to log into multiple Cisco WLCs to find out which controller the newly-added lightweight access points associated with.
One Cisco WLC in each subnet can be assigned as the master Cisco WLC while adding lightweight access points. As long as a master Cisco WLC is active on the same subnet, all new access points without a primary, secondary, and tertiary controller assigned automatically attempt to associate with the master Cisco WLC.
You can monitor the master Cisco WLC using the Cisco Prime Infrastructure and watch as access points associate with the master Cisco WLC. You can then verify the access point configuration and assign a primary, secondary, and tertiary Cisco WLC to the access point, and reboot the access point so it reassociates with its primary, secondary, or tertiary Cisco WLC."
HTH
Rasika
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