A mixer and an audio interface are two different types of equipment, but there can be some confusion between them because they both involve audio signal routing and mixing.
A mixer is a device that allows you to combine multiple audio sources, adjust their levels, and apply effects or EQ. Mixers typically have multiple channels of audio inputs, such as mic/line inputs for connecting microphones and instruments, and often have built-in effects processors, equalizers, and other processing tools. Mixers usually output a mixed stereo or multichannel audio signal that can be sent to speakers, recording devices, or other audio equipment.
An audio interface, on the other hand, is a device that allows you to connect audio sources to your computer, usually via USB, Thunderbolt, or another connection. Audio interfaces are designed to convert analog audio signals into digital signals that can be recorded or processed on a computer. They typically have mic/line inputs, instrument inputs, headphone outputs, and line outputs, and often have built-in preamps and converters for optimal sound quality.
The confusion between mixers and audio interfaces may arise because some audio interfaces include basic mixing functionality, such as level adjustments and monitoring controls. On the other hand, some mixers (especially digital mixers) also have integrated AD/DA converters (soundcards) that allow them to be connected to a computer. However, the primary function of an audio interface is to convert audio signals into a digital format that can be recorded or processed on a computer, whereas the primary function of a mixer is to combine and process multiple audio sources.
In summary, mixers are primarily used for live sound reinforcement and recording, while audio interfaces are primarily used for recording and production in a studio environment. While there can be some overlap between their features, they are fundamentally different types of equipment with different purposes.