Spin Bike At Costco I Want To Get One Of Those Spin Bikes, But Would It Be The Same Thing If I Got A Mountain Bike?

I want to get one of those spin bikes, but would it be the same thing if I got a mountain bike? - spin bike at costco

I could get one of these seats for the rear wheel (I do not know the name). But if I had one foot is the same as a bike ride? In this way I can leave whenever I want. Although it is with the support, in what position?