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I have read in a lot of tutorials lately that the best way to have a super clean mixdown / master is to make sure all your instruments are assigned to certain frequencies.

I am pretty new to producing, and I am just trying to wrap my head around a lot of the more technical side of things.

By assigning certain frequencies, do they mostly mean EQing them where they are most powerful, using spectrum to see where the frequencies really hit, so that the instruments aren't overlapping eachother too much?

Yep, pretty much. You can also EQ them for specific effects, like listening to something far away etc. Basically, the whole idea is to make sure everything can be heard, whatever's going on.

Thank you! Just wanted to clear things up so I didn't get off on the wrong foot.

Yeah you got it right, try to filter out as much unnecessary noise as possible. use high pass and low pass on pretty much everything to cut out unnecessary noise. there are obviously some exceptions, like probably won't high pass sub bass, etc.

Appreciate the responses! Thank you so much.

Try to see where each instrument sits. Say your Sound A hits at 500 hz, and Sound B hits at 1Khz. You might want to lower down/maybe even cut the 500hz frequencies of Sound B and vice versa.
That is, if you don't think Sound B's 500hz frequencies are important for the sound. I mean don't just see where the instrument hits and cut everything else; dynamics are important, too.
Some people choose to sacrifice their 'clean' mixes a bit for making the sounds more interesting, and some sacrifice the dynamics for a clean mixdown.

Try a bit of both, see what you like.