Hey sariah164,
Thanks for using Apple Support Communities. Sounds like you'd like to find out if your Apple Watch can automatically record your spin class as exercise. We'd be happy to help out.
To start, since you have both hands stationary on the spin bike, even though your heart rate is going up, this will not automatically count towards exercise. The same applies if you're out walking and have to push a stroller or something similar. In this scenario, you would want to use the Workout app to track this activity in order for it to count.
When walking/jogging/running, you would want your arms to swing naturally and every full minute of movement that equals or exceeds the intensity of a brisk walk counts toward your daily Exercise and Move goals. In this case it would track that automatically and you wouldn't be required to use the workout app, as long as you're consistently exceeding the intensity of a brisk walk. To provide the most consistency though, we'd recommend using the Workout app.
For more details on these behaviors and what's expected with your Apple Watch, check out: Get the most accurate measurements using your Apple Watch
We hope this information helps out.
Take care!