Both mixing and mastering can be relatively easier to learn when you are being taught and regularly given constructive feedback by someone (or an experienced group of sound engineers) who have worked on hundreds of successful records and not people without a single credit that anyone can look up.
This is the dilemma I think many people encounter. Those with a vast amount of knowledge are busy working on records whilst the average Joe and plain Jane is on YouTube, Instagram, TikTok, Twitch, etc making misleading videos about music production.
I am not going to bother mentioning the need for a good monitoring system, room acoustics, etc.