Radars, star trackers and some math.

Random Google link: https://timeandnavigation.si.edu/navigating-space

Kerbal space program helped me understand a little more of the basics.

The computer aid showing the trajectories helps quite a bit and that's demonstrated quite well in KSP.

As we learned in the Hidden Figures (I did anyway) the math is Euler’s method of iteration. So a desired variable (location of touchdown!) is fixed and then the inputs iterated until a successful set of inputs are found.

you need to play Kerbal Space Program....