A mix engineer sets levels, pans instruments, shapes tone with EQ, controls dynamics with compression and adds space with reverb and delay. The goal is clarity: every part audible, nothing fighting for the same space.
Mixing happens before mastering, which treats the finished stereo mix as a whole.
Using it in a prompt
Describe the mix you want: "clean, modern mix", "warm, vintage mix" or "spacious mix with lots of room". Mentioning a voiceover helps leave space for speech.