They can be part of mixing. They “shouldn’t be,” in that if you’re the producer, and/or tracking engineer, and your client can sing/rap/recite (whatever it is), then you should get a good take.
BUT, of course there is content editing such as “comping” (making one good vocal track out of several different takes), “cleaning” (editing vocal or other tracks to get rid of punch ins or unwanted noise), and “pitch correction” (trying to maximize a sub-standard vocal with various “auto-tunesque” software plug ins).
“Mixing” being the process of bringing all of the tracks together to create a final master mixdown.
Hope that helps...
GJ