Mixing and mastering are two distinct processes in music production. Mixing combines individual tracks (stems) into a balanced stereo file using EQ, compression, panning, and effects. Mastering optimizes that stereo mix for loudness, tonal balance, and distribution across platforms like Spotify, Apple Music, and YouTube.
If you have raw stems, you need mixing first. If you have a finished stereo mix, you only need mastering. For the most cohesive result, having one engineer handle both is recommended.