Both.

Headphones are like microscopes where speakers are like looking through glasses. Best metaphor i can come up with.

But you should always be biased towards monitors, because that's how your music will react in an open environment compared to an isolated aspect.

I use both for different reasons, but you should try to match quality of speakers and headphones.

Speakers relate to headphones, but headphones don't always relate to speakers. (unless you drop like 2grand USD on some headphones, then i'd be expecting some things...)

its very hard to learn production on your own, you need someone with experience and knows more than you, with the technical aspects like synthesis, and chord progressions etc and 'has been there and done it'

so i think its better to look at the online courses, and tutorials and the works of your producers you admired and study them completely

if you can find a mentor, that will be even better because its the habits of a producer that you want to learn from
 
you should be a sponge and soak in all proper knowledge you can

Yeah, it really depends on usecase you think about. If you speak about compession effecting something like an electronic kickdrum then yeah, you can draw automation instead of it. (When speaking about clean non-saturating compressors) ... but the advantage of compressor is that on stuff with "unpredictable" and/or quick transients (real drums, vocals, pianos, synths with a lot of modulation etc...) it will do the trick for you automatically.

So theoretically you could draw "compression-like" automation over a master channel or finished track, but it would take you weeks or months to do it. Compressor will do that for you instantly.

Very insightful feedback! Thank you guys. A few more questions came up to mind when reading through your replies.

1) When cutting out the appropriate frequencies, is it better to use Low cuts/High cuts or Low shelfs/high shelfs? Part of me thinks that using cuts would get me a more clear sound because none of the frequencies would "spill" out. The other half of me thinks it's better to shelf the frequencies and allow it to "spill" out a little more gently so the overall song could be more full.

(This is confusing to me because I don't want my bass synth sub frequency to collide with my sub frequency, so using a low cut seems more appropriate)

2) Do you also cut out the frequencies of the kick within the sub and bass, or is sidechaining it good enough?