It is science but not rocket science. We need to stop trying to make all of this health and fitness stuff more complicated and difficult than it needs to be. Be generally active, minimise sitting time, stand and walk whenever you can. Do some resistance training and vigorous "aerobic" activity. Work hard and progressively on both so that you stimulate adaptation. Rest enough to grow and recover so that you improve. Pay attention to safety. Stay mobile enough to squat and sit on the floor. Diet is responsible for your bodyweight, not exercise.
It is not about the tools
Not mentioned: the tools or the protocols.....because lots of things will work. The tools: machines, bodyweight exercise, barbells. The protocols: one set to failure, 3x8-12, 5x5, Reverse Pyramid etc. intervals or a 400m run. They are not the important concerns. Lift stuff, sprint, recover. Just make sure to pick safe and sensible exercises.
Listen to your Mother
Then there is all the stuff your mum told you: stand up straight, get to bed early, eat your greens, don't snack between meals, stop worrying so much, wear sensible shoes, brush your teeth, get some sunshine and fresh air.
Yes there is science...but really all the focus on the science can be a distraction and an excuse for most of us. The science actually says what I've recommended. The search for the perfect becomes the enemy of the good enough. As I've said before, most of us are not elite athletes either by genetic ability, or profession. For most of us being elite is about maintaining function as we get older. This is not defeatist. Doing these things will get you fitter and looking better than most of those sitting next to you in your office, on the bus or at the pub. Don't stress about the details so much. Get on with your life.