On Agile teams, you typically have 5 layers of planning:
These layers are usually tied to a timebox, or what is often referred to as a 'planning horizon,' e.g., the amount of detail we will go into at this moment is based on the timeline of our goals and expectations. The fidelity of our discussions go from a mile wide and a foot deep for the product vision, to 100ft wide and 1000ft deep for each iteration.
In Scrum, requirements discussions occur at all levels of planning and happen on a daily basis. Cadence-based ceremonies set clear expectations for when the larger discussions happen. In Scrum, you typically have a release planning event every few months, with sprint planning every 2-3 weeks, and backlog refinement sessions ~ 2 or more times a sprint, and then daily standups.
Most Scrum teams start with an iteration 0, which is not a sprint, and during which the team continues to flesh out the product vision, product roadmap, and release planning activities for the first sprint. This is usually done through workshops and activities, over the course of a few days. I recommend checking on Jonathan Rasmusson's The Agile Samurai, as it entails a lot of great material in setting up an Agile team using an 'Inception Deck.'