Mastering requires GOOD ears more than anything plus an awareness of psychacoutic tricks...not to use them but to recognize and avoid them as needed.
A LOUDER master will, initially sound better to most listeners, a master that uses an enhancer/exciter will sound brilliant, initially. Louder, enhanced/excited do often cause listener fatigue and that's a bad, bad thing.

Mastering is the process applied to a final mix - usually a stereo track/file/cut - to have it ready for reproduction and distribution.
For example good old LPs the master included RIAA EQ reduction to allow for the most audio on a side of a disc...bass makes wider waves and can make the stylus jump out of it's groove as well so it's "cut" before the disc is cut and restored in the phono preamp.

Today it's also about loudness penalties - the reduction and compression used by streaming services etc...if done in advance by the mastering engineer or home recordist it'll mean the song won't be as "damaged" by the automatic processing of a given site.
Good M.E.s also make minor adjustments to the audio to make the track sound "finished" for a given genre etc.

I know Bud & Janice's final "masters" sound great - they can use the bits of technology that work for them and use their ears as well to achieve a great result.

I don't trust my ears, (slight hearing loss on one side, tinnitus at different frequencies on each side and a lack of critical listening skills after working on a track for ages), so I save my $$$ and send finished tracks to an M.E. I have a good relationship/communication with and get mine done professionally @ around US$33 a track. Now that I have a solid relationship with the M.E. he often sends me two masters...one that's straight up from what I sent him and another that he's been a little creative with.

He supplies an objective set of ears, experiences and different tastes.

He's also a producer and mix engineer, (been in the business since the mid 60s), so he has experience that can offer a little bit extra. he's also not shy about telling me the mix isn't up to scratch: he'll let me know if something isn't quite right to his ears though he'll do the sow's ear thing IF I want him to.

The Soundcloud and bandlab online things are VERY broad brush strokes and don't involve A.I. let alone ears.

I occasionally "faux master" to get an idea of how my mix will respond to limiters - particularly things like reverb but that's just a checking process, (stacked gentle limiters with EQ between them and some compression). I know less than an half a dozen home recordists that can "master" with a professional sounding result. It's a VERY hard thing to do.

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe