How is it possible to walk over 10k steps, ride a spin bike for an hour and only burn 300 calories???

Total calories is the sum of resting calories and workout calories. Both require that personal information (height, weight, age etc.) are entered correctly. You can check your information at the link below,

Set up and view your Medical ID - Apple Support

You should also add any medications that are meant to control your Heart Rate, under Health Details.

If any of the information above is not correct or not entered and updated, your resting calories will be calculated incorrectly, affecting your total calorie count.

When you walk the 10,000 steps is any of the steps a part of a workout? If relatively few steps are part of workout, they are probably being recorded as resting calories. Your heart rate needs to rise by at least 20 BPM above baseline/resting rate to be counted as active calories. This can result in relatively few calories being burned at the higher, active rate. The bulk of the calories burned are probably from the spin bike, but only if you elevate your Heart Rate.

It seems like that the calories burned are low. Have you tried to calibrate your watch to see if the issues persist?

https://support.apple.com/105048#:~:text=Je%20Apple%20Watch%20kalibreren&text=*%20Als%20je%20een%20Apple%20Watch,in%20een%20armband%20of%20broekband.&text=Wandel%20of%20loop%20hard%20in%20je%20normale%20tempo%20gedurende%20ongeveer%20twintig%20minuten.

You’re welcome. It’s probably related to heart rate not going up just walking at a leisurely pace. Calibrating, as WheelieNick suggests is another good place to improve accuracy.