Here is one where people start. Physical fitness is a building block of a good life. Here are the other building blocks: physically fit, happy, wealthy, wise. The first three, ideally, contribute to development of the last one.
However, once you achieve those skillsets, perhaps you can then recreate the other states.
I would consider that, having 'leveled-up.'
Physical fitness consists of two basic things: exercise and diet. Despite all the complications of what we read about, exercise and diet are the two basic things.
People complicate these two things in order to sell a new product. If you notice, most websites, all the products and content, basically revolve or have one idea at the back of it. The idea of this website is that every smart person should be able to figure out how to be happy.
Physical fitness is using your body, pretty much everyday. You have cardiovascular system and muscular systems. Both are good.
The foundation of building both physical fitness and diet are the macro system of knowing how to start and maintain and project.
Once you can figure that out, then you can have a more robust understanding or system of physical fitness.
That said, having it be completely robust is foolish, it should be adaptable, not rigid.
The basics are exertion, difficult, probably 2-3 times a week, with cardio or other lower impact everyday.
Diet should function to support the output of energy you have going on. That could mean, primarily, good meat (consider hunted meat), an abundance of vegetables, and some fats. Lots of diets consider cutting out carbohydrates. Those could be true. It depends on a synergy between diet and exercise, again, so if you are eating the diet of a manual laborer, but your exercise level more closely matches someone sitting in an office all the time, you are developing a problem.
There are entire websites and books dedicated to details of both.
For exercise, the basics are that you need cardiovascular health which is built from various long, slow exertion, medium exertion, and high exertion running. Then there is muscular movement.
I like running, walking, and ballistic body movements. Flexibility is also key as injury prevention and also to increase performance. Over training should also be avoided.