the key to all of it

Hmm. Is there really a "key to all of it"? Yes, there is.
Repeat after me: Progressive Overload
Let's talk about that as it pertains to building or maintaining bone, muscle, red blood cells, and mitochondria (your metabolism).

Bone

Strength training stimulates bone formation, which is a complex process involving bone cells acting as tiny sensors, detecting stress and signaling for repair and strengthening.

Living and walking in Earth's gravity stimulates bone formation too, which is why astronauts lose bone density rapidly when in space.

What if your hormonal changes, aging, and other factors have you losing bone faster than you are repairing it? You will get a diagnosis of Osteopenia or Osteoporosis.

This is where progressive overload comes in. Maybe you were maintaining your bone density. Good job! That means the level at which you were exercising was working. If your bone density starts to decrease, the exercise prescription for that would be to safely increase the amount of weight or reps. You have to put stress, or load, on the bones for them to sense the need for new bone. It's time to level up.

Muscle
Lifting weights also triggers a signal that stimulates muscle growth. To trigger this signal, we have to subject muscles to enough tension and stress to stimulate muscle protein synthesis. This requires - you guessed it - progressive overload.

Notice it isn't called load. It's called overload, which can be done with more weight, more reps, higher frequency, or higher intensity. In other words, we have to push ourselves just beyond what we are currently capable of. It is the only way to adapt.

Red Blood Cells
The formation of RBC's is near and dear to my heart as a kidney patient who has been through two bouts of anemia that felt like the end of the world.

Luckily, they make a synthetic version of the hormone that stimulates new RBC's called EPO. Do you know what else stimulates new RBC's? Progressive overload.

That looks like going into higher elevation places where oxygen is less available. That stress, called hypoxia, stimulates the production of the hormone EPO, which tells bone marrow to get to work. New red blood cells needed. The higher you go, the more RBC's you make.

Mitochondria
We all want more mitochondria, whether we know it or not. They are the energy centers of cells.

I thought about listing all the amazing things your mitochondria do, but let's just say when we talk about our metabolism, we a really talking about our mitochondria and their ability to burn fuel and create energy. They rock.

Once again, mitochondria creation, aka biogenesis, is triggered by cellular energy demands and stress.

I looked up stress in this context:
"Stress is the physical challenge or stimulus placed on the body during exercise that disrupts its normal state, or homeostasis. To build strength and muscle, the body must be forced to adapt to this new level of tension that is above and beyond what it has previously experienced."

All four systems work the same. Above and beyond.

Provide a level of stress that is enough to trigger the adaptation. The body does the rest.

I know that can sound daunting. Or even unsafe. That's why there are trainers, and it is what we are specifically taught to do. We design plans for progressive overload that are safe for you. Like I have said for years, we are finding that place between challenging and doable that makes all this magic happen.

I've seen it work over and over. Those lab results change. The muscles develop and get stronger. Heck, I have so many red blood cells now after hiking at 10,000 ft this summer, I don't know what to do with all this energy!

I guess I'll go create your plans for the week. Be ready. We are going to LIFT!