This is the moment where the pre production and recording stage come together.
The performers already have done an amazing job on playing their pieces, the recording engineer captured the sounds at the best possible quality and the mixing engineer is now happy to continue working towards a final product.
Mixing can be a fast and dynamic process where the sounds remain intact and not much heavy processing is being used to get the desired sound. But it can also be a very tedious, surgical task to get something sounding right and sits where is needs to be.
Parallel processing, vocal tuning, drum replacement, using effects, automation and the use of tape are all methods to get exactly what the song needs.
Tons of plug-ins are at disposal. One is not particularly better than the other, one just needs to know how they work and how to use them when needed.