The best way to learn cabinet making is to sign up for a City & Guilds at a decent college like Chichester or Rycotewood. Failing that complete a paid for long-course at a respected workshop like Peter Sefton or Waters & Acland. The huge advantage of this type of training is that there's an experienced man at your shoulder, and just as importantly, there's a disciplined approach that takes you through a structured syllabus and won't let you drop into all the rabbit hole diversions that fascinate woodworking newbies.
But for most people these aren't practical options, so if they want to learn to make furniture they'll basically have to teach themselves.
To stand a decent chance of success they need a single primary source of information. There are many different ways of getting the job done in furniture making, but mixing and matching different techniques from different people is usually a disaster, especially if the people dispensing the wisdom are internet know-nothings who are long on opinion but short on practical knowledge.
Paul Sellers is one of the very, very few candidates able to fill the role of primary mentor. Yes, many of his designs are stuck in the 1970's, and yes he's prone to sounding like a new age messiah. But that really doesn't matter, he'll show you techniques and skills that work, and he'll take you through a structured series of projects that will equip you to build good quality furniture in your own workshop.
So if it's as easy as that how come so many hobbyist woodworkers spend a couple of years banging away in their workshop but still haven't made much progress at the end of it?
The problem is that many woodworkers just don't have the discipline to knuckle down and spend ten hours a week for two years precisely following Paul Sellers instructions and doggedly working through all his projects in exactly the order he recommends, using the same tools and equipment he works with. And of course they also end up falling into all those fascinating rabbit holes like sharpening, fettling non essential tools, or home brewing exotic finishes.
If the objective is to just occupy a few hours by tinkering in your shed then of course there's nothing wrong with doing whatever takes your fancy on any particular day. It's your money and your time, so you're free to spend them any way you like.
But if you genuinely want to learn to make furniture, and you're unable to sign up for extended full time training, then there's very few alternatives to Paul Sellers and a big dose of self discipline.