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Answer
Classic Bluetooth uses a codec called SBC. Its been used in BT classic devices for a very long time. Its also mandatory for audio applications, providing a common codec that all BT classic audio devices have access to, ensuring interoperability.
Other 3rd party codecs are also available such as aptX and AAC amongst others. These 3rd party codecs often provide improved audio quality or reduce latency but because they are no mandatory in the BT specification not all devices support them. Some are even proprietary to specific silicon vendors.
LE Audio introduces a new codec called LC3. LC3 gives developers options. With LC3 you can get the same or better audio quality at lower bitrates than with SBC or you can use higher bit rates and get even better quality audio.
A lower bit rate will result in less time on air and therefore potentially lower power consumption, which may be important for a battery powered device. Alternatively developers can choose a higher bit rate for maximum quality.