I want to create a fantasy world as fully detailed as Robert Jordan and Tolkien.
This is an interesting question/statement because my thoughts on world-building have to do with making sure a contemporary world "which includes your characters" stays consistent. This question deals more with epic fantasy like the Wheel of Time and the high fantasy of Middle Earth.
Middle Earth is fascinating. There's a lot there, lots of background work, lots of connections, enough to fill a text book many times over. Both Jordan and Tolkien had a strong interest in mythology and history, and the ability to use what they knew to create something totally fresh, but what they didn't do was a "from scratch" creation. More like borrowing. There's Anglo-Saxon, old Germanic influences, Lang's fairy tales, Viking sagas, Norse mythology, and military/religious theories, in other words, a mish mash of everything each man knew, he threw into the pot. They both had the framework of "how" a world should operate and work, they simply filled it in, like a rectangle to which they added puzzle pieces.
What makes it epic is the structure. An overall arc like in a trilogy where each book is part of the greater whole, but also separate in itself. The "story itself" arcs over the continuing individual arcs of each character or group of characters in multiple books. Like a cake with lots of layers.
Each layer is separate, but they touch and the outside frosting holds it all together. In other words? It's not really a book, but a bunch of novellas with a common focus. Or a movie where the camera cuts away from an actor to follow the action somewhere else before swinging back to what the movie is really about.
Do you need to have the world in place first? Yes and no.
World-building is fun, but there's no point in creating Australia if you never mention Australia. If somewhere, sometime over the course of your writing, you "might" mention Australia, start an Australia file. Get a notebook, or open a folder, and start collecting bits and pieces. By the time you get to it, there will be enough Australia there to create a continent and people. Writers are like dragons. We hoard stuff. Bits of dialog, a river, odd coins and tree-colors. The story is important, because it influences what you need to build your world.
Build what you need.
If your hero is going to cross an ocean, you need to create an ocean and put something at the starting point and the end. You need stuff in the ocean, colors and smells, clouds in the sky and people on the boat. Are the people important enough to mention? Are you going to tell your reader about them? Then who they are, where they come from and everything associated with their culture "in regards to that person" need their own separate file/page. If they're going to talk about Tavar shampoo. It helps to know why it's important and where it comes from, how much it costs, and how much a Tavarian coin is worth in your heroes currency.
Everything interlocks, but "everything" doesn't need to be there. The size of the ocean is good, but if a little fish exists on the bottom of the Tavarian Trench and no one other than the author knows about it, it's a waste of time.
A story bible--the background--can expand. Files get added, rules become common usage, if a tavar is worth three kronor and the captain is Lyssian, maybe he uses gold, so your world might be able to use moneychangers. Thieves would know people frequenting a money changer would have money, so now we have thieves, and some kind of crime. That means you need a map of town so you can see where the safe areas are, and where not to go. It all interconnects in a way that directly influences your people and plot.
So world-building yes--maps and stuff, applicable cities and people and money and attitudes. All the necessary bits, an encyclopedia? No. Not unless you personally know how everything works. Most people don't. I know if you do "this" thing you get "that" result. But I don't know everything and your hero doesn't have to know everything too.
You need basic rules, like the water is wet. People drown (or don't drown as the case may be) Magic works "this" way (however it works in your world), wizards wear pointy hats (or don't). The King is the boss. But if there's a Royal Holder of the Lace Handkerchief and your hero never goes near the Lace Handkerchief--then it's just procrastination. Fun procrastination if that's what you want to do, but still...