You'll want to study hypertrophy and how the two types of load have different effects on different types of muscle.
You failed to state your training goal, so, it's hard to advise you in any way. Riding a bike under load, in high gear, can give you some BIG legs. Riding a bike in a lower gear, at a higher RPM, will not give you substantial development, and won't really up your cardiac threshold. Bicyclists that are serious often develop huge legs from progressive overload. Wanna' bes that are lazy have skinny legs. I had my largest legs, almost 29 inches (when I road my bike in high gear). If you look at some Olympic speed skaters, and cyclists, you'll see many have very large thighs.
If size is your goal, higher weights, at moderate reps (8 to 15, or more) will cause more sacroplasmic hypertrophy. If cardiac threshold is what your after, HIIT, will raise your cardiac threshold, and increase your strength, and local muscular endurance. I.e. sprints, and intervals. If just burning calories at a low intensity level is what you want, low intensity, high rpm, riding is where you're burn calories (but, muscle, too). For strength, and myofibrillated hypertrophy, you'll do low rep, high weight, exercises...think powerlifter (not much size this way, but, more strength).
Spinning will burn calories, and once your body gets you used to it, may actually lower your LBM in the rest of the body if you don't have enough calories.
Size is about calories, and progressive load, at moderate reps, consistently for sacroplasmic hypertrophy. Stength is about 1RM, 3RM, and progessive loads, but, much longer recovery, and not much hypertropy other than myofibrillated. Fitness is about HIIT. Calorie burn is about low intensity cardio, although, study after study has shown HIIT to be very much more effective than endless ours of cardio, and it increases lean muscle mass / preserves it, and increases cardiac Thessaly at the same time.
Resistance training is the only training that will increase your lean muscle mass, improve your cardiac threshold, and strengthen your bones, as you grow older.
Calories are key for success. If you don't eat, you're wasting valuable time, and effort.
Higher rep and long term endurance movements help with capillarization and vascularization which help to make an improved blood supply to a particular muscle group. High rep training should be integral to any serious athlete's
training. No serious athletes should blow past HIIT. It's a huge contributor to cardiac health. (I'm considered at ZERO risk of a cardiac event.)
You'll want to study this more on your own, to consider what your goals are.
Is riding a bike like doing squats? No way! Think about it for a minute. When you squat you involve your back, your butt, your arms, your chest, your shoulders, your hamstrings, your abs...and it's work. If had thought about it, that would almost certainly have been apparent to you. There's a lot of proprioception and the resulting balance that goes into a squat. It's called the king of exercises for a reason.