We can't actually see the exoplanets directly.

What we see is a slight dimming of the star it's orbiting as it passes between the star and us. Imagine someone walking in front of a car with it's headlights on while at night. You can see the change in the light, but it's unlikely you can see the person. Same principle.

We can then look at how the star "wobbles" in it's orbit. That gives us an idea of how much mass the planet has.

As a bonus: With the right tools, we can look at the light just as it starts to dim and this will give us an idea of what the atmosphere is comprised up.