here is my stance on these things and it is backed by actual human studies and history. anyone who wants to argue this with me can take a look at the best distance swimmers in history (i'm not talking about the jagoffs who swim 3 laps in a fucking pool), they all were 20%+ body fat and they do more cardio in 1 swim across the ocean then most do in 5 years. the dude who swam from LA to catalina island was 22% body fat and that is one of the most brutal swims you could do
cardio: builds heart health and endurance
weight training: breaks down muscle and stimulates muscle building properties
diet lifestyle: this dictates how lean or fat you are
homeostasis: this is a huge factor and hard to overcome. if you been fat for 20 years it is hard to reverse that. if you been skinny 20 years, fell off the wagon for a year, then want to get back in shape it won't be hard
genetics: they did a study in europe where separated twins who went to fat parents and skinny parents. the twins still ended up both fat or skinny no matter what their adoptive parents were. can this be overcome? yes but it takes a lot more than what the cookie cutter diet coaches and PT's will want you to do