What you are training
Masking is why a mix can be clean track by track and muddy as a whole: two instruments sharing the same frequency band, each hiding the other. The vocal disappears under the guitar; the bass and the kick smear into one thump. Fixing it is easy once you can hear it, because the cut goes on the instrument that matters less in that band. Hearing it is the skill, and it is one most people have never practised in isolation. This drill plays a short arrangement, soloes nothing, and asks you which two elements are colliding and roughly where. The reveal shows the overlap, so you connect the muddiness you heard to the band it lives in. Six rounds train the listening habit that separates a mix engineer from someone who owns the same plugins.
After this drill, the plan: The daily plan drills masking on full stems next, where you make the cut yourself and prove it at matched level.