Moves
Each playbook has a selection of moves. You'll start play with 3 or 4 of them; your playbook tells you which, or which to choose from. Each time you Level Up, you get a new move from your playbook.
Some playbook moves lack triggers; you simply gain their benefit when you take them. If you take the Improved Stat move, you permanently increase one of your stats. If you take a move that lets you always ask something, you can just do that now!
If a move says "(Requires __)," then you must meet that requirement in order to take the move. The requirement might be a level, another move, or the specific playbook (meaning that other playbooks can never take this move, even when they take a move that lets them pick from another playbook, see below.)
If a move replaces a different move, then it requires the one it replaces. When you take such a move, you lose the original move and any benefits that it conferred.
Many playbooks include a move that lets you pick from another playbook. Most of these moves limit which playbooks you can pick from.
If you really want to take a specific move from another playbook, but you don't have an option to do so, then pitch your idea to the GM and the other players. If everyone agrees, okay. But there are reasons why the playbooks have access to the moves they do. Magic-oriented playbooks have less flexibility when picking from other playbooks, and their moves aren't as freely available to others.