My old router from BT allowed me to choose 2.4ghz or 5ghz on my devices, however the new router ( smart hub ) seems to do it automatically ?
It is the device that selects which one to use not the router but if you mean you could split the 2.4Ghz and the 5Ghz on the old hub but not on the new one you are correct.
Yes if it is all that the device can use. If it can use either will select the strongest signal.
Some devices let you set them to only use one or the other or both signals.