An idea is a topic (e.g., "how to bake bread"). A concept is the specific execution, angle, and packaging (e.g., "Baking Bread with ZERO Tools to see if it's actually possible"). Concepts are what get clicks.
Test it against the "Why would anyone care?" rule. If you can't answer that in one sentence, the concept needs more work. A good concept usually solves a problem or satisfies deep curiosity.
Yes! Many top creators design their thumbnail and title BEFORE filming. If you can't imagine a compelling thumbnail for the concept, the video might be hard to market.