Solution based requirement elicitation happens when your stakeholders are sure about the end state/goal. Stakeholders are very much sure of the business problem that the end state will resolve. In such a scenario, the primary objective of a BA is to gather requirements to understand how to achieve the end state.
First step is to understand the business problem. In your e.g., if I need a Fitbit, why do I need it? What problem will it resolve? A BA should ask questions to identify if I need a Fitbit because all my friends have it or I need a fitbit to make sure I keep a track of my fitness activities or I need a Fitbit to ensure my motivation towards my health or something else.
Upon identification of the problem, next step is to identify the reasoning behind the solution. Why Fitbit? Why not Garmin or something else? Why only Fitbit can resolve the identify problem? Is it becasue the FitBit has more features or is it because of the cost or is it becasethats the only solution I know?
Upon identification of problem and the reasoning behind solution, a BA can then jot down the approach towards a given solution from a business problem. Start with questions like who is going to benefit out of this solution? Will you be wearing Fitbit whole day? If not how many hours? What are your primary hours? How long charging would be needed and so on and so forth.
Once approach is decided, start asking questions on solutions and design. Ultimately you'll figure out everything a BA needs to.
When I interview a BA, I like to ask them what if you need to open a restaurant in a food court. How would you elicit requriements and how'll you identify the stakeholder's ask?