Sorry, I'm a high schooler quite new to the mathematics behind astrophysics, so if you can keep it ELI5 I'd appreciate it! I'm currently in AP calculus, so you can use mathematical terms.

I'm actually trying to learn how to calculate the rotational velocity of galaxies (Milky Way in particular) and I'm a bit clueless :(

So my questions are:

1. How does the most basic angular velocity equation compare to calculating the angular velocity of the Milky Way galaxy?
2. Are the calculations of all galaxies similar (same physics, same process of solving, same equation)?
3. How does dark matter affect the calculation? Or does the angular velocity stay the same with the effect of dark matter on the rotation? I would assume that dark matter would cause a certain star to rotate faster at some points, but is this completely wrong?
4. When calculating the rotational velocity of the Milky Way galaxy, are we looking at the movement of one star close to the center or at the edge of the galaxy? Does the angular velocity of the star's rotation stay consistent throughout?
5. How do we know the size of the galaxy and the movements of a star within the galaxy?

Thank you very much!