Bluetooth dual mode (BR/EDR + LE)
Bluetooth Basic Rate/Enhanced Data Rate (BR/EDR) is typically used for relatively short-range, continuous wireless connection such as streaming audio to headsets.

Bluetooth low energy (LE) is designed to use short bursts of longer-range radio connection, making it ideal for Internet of Things (IoT) applications that don’t require continuous connection.  These apps can often run on just one coin cell and still have a relatively long battery life.

They use the same radio spectrum, but different channel allocations.. Bluetooth BR/EDR: 79 channels, channel index from 0 ~ 78, covering from 2400 ~ 2483.5MHz, with a bandwidth of 1 MHz. Bluetooth LE: 40 channels, channel index from 0 ~ 39 covering from 2400 ~ 2483.5MHz, with a bandwidth of 2 MHz.  Three of the channels are advertising channels as shown in green.



Bluetooth radios do not stay on one channel continuously, but instead use adaptive frequency-hopping to avoid interference.  The hops from one channel to the next take place 800 times per second (every 1.25 ms).  The "adaptive" part means hops are only made to uncrowded channels if possible.

Dual-mode chipsets exist, but they are like having two different radios combined into one -- sort of like an AM/FM radio (not a great analogy though since AM/FM uses completely different frequency bands).

Besides using different channel allotments, classic Bluetooth and BLE also use different protocols.  Streaming audio protocols such as A2DP, and serial port protocols such as SPP are only available with classic Bluetooth, for example.

Depending on the chipset, both modes (BR/EDR and LE) can be used simultaneously.

For further information, you could consult the Bluetooth SIG, which has lots of pages like this one covering these topics.