I hope this isn't a noob question, but how are they done?

Uhh… they report true or false, depending on if the condition is met.

<key space pressed?> -If the spacebar is held down, report true

<color ██ is touching ██?> -If the first color is in contact with the second, report true

<not <mouse down>> -If the mouse is not being held down, report true

<loud?> -If microphone volume is >30, report true (only in 1.4)

([variable1 v] of [Sprite2 v]) -report the value of the local variable of Sprite2

(current [second v]) -report the current second of your computer's clock

Learn more at Sensing Blocks.

No, as in the “Underwater life” project.

Don't know what “Underwater life” project is but there's stuff about using sensors in the sticky topics.

Check out my project here. It has a slightly visible sensor when touching black. How it works is that each of the four lines on the sides of the marble are a slight tinge of a color (enough for a distinction between sides, but barely enough to be invisible), and each line detects when it touches black (the platform) it prevents the marble from going through the platform.

Hope this helps!