I wish you all a bright day! I have a demo song called “Serious Effect” by Michael Jackson. It’s almost there already 'cause MJ was a perfectionist himself, but I’d like to turn his demo to a modern music style with the highest quality as he’d always done. This version’s intended for strictly personal, re-creative use only (no commercial release from my side). I’m the only one interested in MJ among my family and friends, so the file’s safe.
That’s a “big question”… There are books written on this stuff!
The “final step” in a professional recording is[u]Mastering[/u]. That’s the “final touches” after mixing such as loudness adjustments and maybe some slight EQ adjustments, etc. Even if you aren’t doing anything else it’s helpful to make the level adjustments as a separate step after mixing. Modern mastering usually involves compression and limiting (with make-up gain) to bring-up the loudness without clipping/distorting.
In the pro world mastering is done by a different person with “fresh ears”. But of course with most home recordings" you are the performer, producer, recording engineer, mixing engineer, and mastering engineer…
The quality of a recording depends on the “links in the chain”. It starts with a good performance on good instruments. Then you need a good recording with good equipment and good mic placement, etc., ideally in a soundproof studio.
The mixing engineer mixes (blends) the various instruments & vocals, while often using effects (EQ, reverb, and compression are common) for a good overall sound. Effects can be “corrective” (to correct for an imperfect microphone, etc.) or they can be “special effects” to make unnatural but interesting sounds.
Note that mixing is done by summation (analog mixers are built-around summing amplifiers). So you have to be careful not to exceed 0dB and clip (distort). More advanced software (and hardware mixers) have level controls for each track/input, plus a master level control. Audacity doesn’t have a master level control so you have to reduce the individual tracks, and if you have lots of tracks you’ll have to reduce the levels a lot. But if the overal mix ends-up too-low, you can boost as a separate mastering step after mixing & rendering (exporting).