What is the difference between mixing and mastering?

Mixing and mastering are two jobs, in that order. Mixing decides how the parts sit together. Mastering decides how the finished song sits next to other songs.

Mixing

In Take One, the mix is the vocal fader against the instrumental fader, plus EQ, compression, and effects. That work is included with the free download. You are shaping a relationship: voice against everything else.

Mastering

Mastering is the last pass on that finished mix. The Master screen measures loudness against common streaming references and applies true-peak protection. Mastering is part of Studio. Use it as a guide, then listen, and check the delivery spec you were given.

Which one first

Mix first. A master cannot repair a vocal that is buried or shouting. Master last, once the balance is one you would actually release. The practical steps are in how to master a song.