There was a Kurzgesat video about this a couple weeks ago. Apparently if you don’t spend calories exercising/biking, your body will find other ways to burn it like increasing your immune system activity (which can have poor long-term effects). There’s an adjustment period when you do start exercising where energy is still spent on sedentary things and the actual exercise before the former is reduced to mostly match the latter.

I have also read that regular exercise can lead to an increase of base metabolic rate by ~5% though, which is like an extra 100 calories per day.