The Core book is like the D&D players handbook. It’s the starting point with the all the basic rules, races, classes, items, spells etc. There are some monsters in there to tide people over while the game is new but not really enough. All the other planned books are expansions to the core book not replacements. There’s also a lot of cool lore in the Core book for the Great Parch and due to the inclusion of Har Kuron in the Anvilguard book it appears the game is going to keep up with all the advancements in the main Age of Sigmar plot. Which is why people are speculating over the Champions of Death book and its implications.
There is a starter set coming https://www.cubicle7games.com/product/warhammer-age-of-sigmar-soulbound-starter-set/ . It gives you the very basic rules to use and set characters with a newbie adventure, but the core books goes into a lot more detail and has the character creation rules.