Equalization. How, when, why, what, how much?
OK, a short "rant" about EQ.
Charts like this (see photo above) can be enlightening if you have no musical training but they don't answer the question "how, when, why, what, how much?" at all... and it can't. Musical notes are frequencies, then you have harmonics and how the "stack up" to form the tone and sound of the instrument (which is influenced by the shape and material of the instrument, just like... wait for it... all rooms / spaces!)
Then you have a musical composition which can be in any of the 12 keys... either one throughout the song or not in any key (or something in-between).
You have (hopefully) the arrangement and the parts/lines that the musicians play on the instruments that the arrangement is made of.... and that will first and foremost give you the frequencies of the recording.
Then you record... and you have mics, position of the mics, and space, phase and polarity ... all which contribute to the tonal color / frequency balance. Then you have equipment that might or might not color the tone further.
So ... what do you EQ and why? Cut out problems? Enhance the "good stuff"? Create tone, create character/vibe? Make the arrangement work better? "light and shade"...? ....this is all "program depended", artistic vision based or a technical problem solving decision (or all of the above) as you might realize by now and you can't give a single "stock" answer to such a simple question as: "What do I boost and/or cut on a given instrument?" To add even more factors to the equation, then your monitoring (what you listen on, headphones and/or speakers) and the room your in (if you're listening on speakers) also play a HUGE role in how you hear frequencies... because, remember... all spaces have a "sound" or a tone... their own frequency "sweetspot" if you like. I mean, just try blowing into a bottle, tapping on a glass or a bowl.... they most likely resonate (create a tone ( = frequency) based on size and material. If you've ever played a bass instrument then you've most likely also experienced how different rooms can drastically affect your tone / amplification (how it sounds basically). This applies to all instruments but is more likely to be problematic on bass instruments as lower frequencies travel slower and have the tendency to "build up", i.e. in corners.
So, as you can see, there are many many contributing factors that add to the decision that a audio engineer might make when he decides how he's going to EQ a given instrument. The act of EQing can be a simple one though... give or take... depending on your experience and monitoring, but how you came to the conclusion to "make that EQ change" ... depends!