Simple, it only has one radio. One radio === one channel. If it has two (or more) radios, then it can use one for the mesh, and the others for clients.
If your WLAN employs security, then every device on the WLAN needs the correct settings (i.e. password) to connect.
Note: Dual-band does not mean "dual radio". And it only has two internal antenna.
At layer-1 and layer-2, Wi-Fi is used instead of a wired protocol like ethernet.
Above that, the layer-3 protocol (IPv4, IPv6, etc.) will be the same, as will the layer-4 protocol (TCP, UDP, etc.).
Protocols above layer-4 are explicitly off-topic here but they will be the same as used on a wired network.
The whole idea behind the network stack is that each layer in the stack is independent of the other layers in the stack. For instance, IPv4 happily runs on many different layer-2 protocols (ethernet, Wi-Fi, PPP, frame relay, HDLC, etc.), and it doesn't know or care which layer-2 protocol carries it. Layer-2 protocols, such as ethernet and Wi-Fi, can carry any number of layer-3 protocols (IPv4, IPX, IPv6 AppleTalk, etc.), and they don't care which layer-3 protocol they are carrying.
As far as an application is concerned, nothing has changed, because the application is oblivious to the lower-level protocols being used.
Best Answer
Your client has a radio which listens to the wifi 802.11 frequencies for the country configured in the radio.
Wifi APs are configured with an SSID; those SSIDs are known via beacon frames; by default beacon frames are sent every 102.4 milliseconds.
The beacon frame format includes an SSID field, and the beacon interval.
The radio in your AP is configured for a specific channel. Those beacon frames are transmitted on the AP's channel.
Every wireless AP can select from a pre-defined list of wireless channels, and it chooses one channel to announce the SSID. Your wifi client constantly walks (or scans through) all those channels to figure out which SSIDs are available.