Choosing the best AP and band is a decision made by each client device. There is no 802.11 standard for AP selection, so each device is unique in how these decisions are made.
You can confirm which band and which AP your client device has connect from: [Manage > Clients]
You can also manually select which AP to connect to from the client by enabling the use access point name option from the SSID configuration options: [Configure > SSID X > Use Access Point Name]
You can also manually set unique names for an SSID using 2.4 GHz vs 5 GHz. [Configure > SSID X > Band: 2.4 GHz | 5 GHz | Both - Unique SSIDs]