Q=HR x SV
Q is cardiac output: how many litres of blood your heart pumps around your body. At rest this is about 5L
HR is heart rate: how many times your heart beats in a minute, on average about 72BPM
SV is stroke volume: how much blood your heart pumps out each time it beats, on average about 70ml
Q (cardiac output) is constant for any given workload. Eg at rest the body will need about 5L of blood per minute, as you exercise, your body requires more blood, so it will increase both SV and HR. SV will increase by about 50% while HR can increase up to about 220-age (it can go higher but for physiological reasons it becomes inefficient).
Depending on the exercise, the heart can change in one of two ways, first it can grow in size, with a larger overall volume due to an increase in ventricular size (eg it can hold more blood), if it can hold more blood (say 100ml at rest rather than 70ml), our HR will come down; Q(5L)= HR x 100(SV), therefore HR = 50BPM.
Next, the size of the muscle fibres can grow (normally more through resistance training) which increases the power of the heart muscle. By having more power, it can push blood through the system easier, meaning the heart doesn’t have to work quite as hard. Think if you were trying to drain a pool, would you want a pump that came out of your mums decorative water fountain, or a fireman’s hose pump? The fountain pump would break before the job was done due to strain.
So the fact the heart is beating less, and not having to work quite as hard to push the blood through the system means the heart is healthier.
Now take into fact that exercise also has benefits of reducing lipids/triglycerides (fat) in the body, artery stiffness decreases (lower risk of stroke and clots), improved VO2 (ability to perform work), reduced risk of cancer, better mental health and performance, among a plethora of other advantages, not including benefits that go along with exercise (people that exercise are more likely to eat better and not smoke) you get better longevity outcomes and quality of life.
In saying that, at a certain point, you can over do it and cause damage. Elite athletes have higher mortality rates due to the prolonged stress they put on their hearts (by not allowing the body to rest fully, it doesn’t repair, heart included) along with the drugs and steroids they take including testosterone, EPO( increases red blood cell count= higher risk of clot), HGH (heart and organ swelling). They also have the risk that their HR can drop so low that they pass out (some athletes have been recorded to have resting HR as low as 20BPM during sleep).
TL;DR exercise improves heart size. Larger heart can pump more blood per stroke. Heart doesn’t have to pump as hard or as often to move same amount of blood around. Lazy you reaps the benefits of what sweaty you did in the past. Go exercise
Source: exercise scientist