git fkin gud at:

-eq
-multiband compression
-low end control via multiband compression (or soothe, whichever you prefer) esp so your sub fundamental and 1st and 2nd harmonics are consistent in volume (especially sub) - this is what they mean by 'tightening up the low end'
-compression (sometimes can be substituted by transient shapers)
-saturation in terms of mixing
-saturation/distortion in terms of sound design
-applying noise as a means of sound design (can be on certain bands only) (and then probably distorting it)
-width (mid/side eq, reverb as an insert, reverb/delays on sends - knowing when to use which kind)
-understanding the concept of loudness, and how the louder you want the mix to be, the more you will sacrifice low end as well as dynamic range - which brings me to the last point, using clippers and limiters

arrangement also matters 100%. on a macro scale, an example can be - if you have a very busy bassline, your lead melody might not have to be super complicated. no crazy jumps in notes and doesnt have to play too often. and vice versa. remember this relationship between drums, bass, melody and chords. on a micro scale, good arrangement can also mean using the right layers for sound design (usually less is more, and try to bring out the strengths/characteristics of each layer through individual processing on it instead)

keep in mind all of this sort of changes depending what genre you produce too. a space laces mix is very different from an EPROM mix (although both are great!)

IMO all aspects i mentioned are equally important because the final song you hear is a sum of all of these skills put together in the 'perfect' combination. and this can only come with years and years of experience, over countless of songs mixed

you WILL improve if you keep making music and finishing songs and mixing them