First the jargon:
'Stems' - in music production, a 'stem' is a set of individual unmixed, unmastered audio tracks (eg bass, drums, vocal) which can be mixed in an audio workstation. Artists may create 'stems' for other people to mix, or remix.
'Atmos mix' - this refers to a mix of music for Dolby Atmos surround sound systems. Surround sound uses several channels, as opposed to standard stereo which just uses two.
A lot of the major label projects involve working from stems and matching the Atmos mix very closely to the stereo master, but in immersive space.
Major labels (record companies) apparently work from stems, ie artists provide them with unmixed stem files for their producers to mix. Because they have the unmixed, unmastered audio tracks they can mix them in different ways. They apparently create Dolby Atmos mixes which are very close in sound to the stereo mixes of the same work, but they aim to make them sound 'immersive' (surround sound creates feeling of 3D audio because the speaker placement adds the dimension of height).
This can be a challenge when stems are not created carefully or have been mixed in a top-down workflow, and that part just removed for mastering/stem creation.
This suggests that stems should ideally be created with surround sound mixing in mind - that process is what they call a 'top-down workflow'. But it suggests that many stems are just tracks removed from a standard stereo mix.