The Four Foundations of Mindfulness are a basic blueprint of the teachings of the Buddha.
The body contemplations, as well as calming our mind, help us to create spaciousness around experiences so that we can observe them non-judgementally and without attachment. The elements meditation helps us understand anatta, not-self, and interbeing. The body parts meditation helps us see unsatisfactoriness. Meditation on mortality helps us realize impermanence so we can let go.
The next three Foundations of MIndfulness: feeling tone, the mind and Dhamma mainly teach us about impermanence, fading away of attachment, cessation of suffering and letting go.
Thus the Four Foundations of Mindfulness are all geared towards showing us the three characteristics of all conditioned things: impermanence, unsatisfactoriness and not-self. They also counteract the four Vipallasa, the distortions of perception. Seeing the three characteristics leads to liberation.
We do our best to respond to life's situations by being fully present and then we let go of expectations and the delusion of control.
This is all we need to focus on in our practice. Nice and simple. We just need to do it! The sense of urgency helps motivate us to do the practice. We cultivate this with the contemplation of mortality.