You are asking some generalized questions; without more specifics I cannot comment more than guess.
Here are some good places to start with the math that I found online for you:
Trigonometry for Game Programming by Matthijs Hollemans
https://www.raywenderlich.com/35866/trigonometry-for-game-programming-part-1
https://www.raywenderlich.com/35944/trigonometry-for-game-programming-part-2
(These tutorials quickly get pretty heavy, but they have wonderful pictures explaining stuff. You can safely ignore all the stuff about Cocos2D.)
Also, Khan Academy has a pretty good video tutorial explaining right triangles and the Sine/Cosine/Tangent functions:
https://www.khanacademy.org/math/trigonometry/trigonometry-right-triangles/trig-ratios-similarity/v/similarity-to-define-sine-cosine-and-tangent
(Unfortunately, the author sometimes mumbles words, but you can follow the transcript below if it helps you.)
Linear Algebra for Game Developers at Wolfire.com
http://blog.wolfire.com/2009/07/linear-algebra-for-game-developers-part-1/
http://blog.wolfire.com/2009/07/linear-algebra-for-game-developers-part-2/
http://blog.wolfire.com/2009/07/linear-algebra-for-game-developers-part-3/
(Part 2 has the information about dot and cross products.)
Now, as for your specified concerns...
Make sure you understand the concepts of
vectors and a
coordinate system.
A vector can be treated as the far point of a right triangle in your coordinate system!
I'm not sure why you would need the dot product to reflect a projectile. You only need the angles of the wall's “normal” and the incident projectile.
The normal is a vector that is perpendicular to the wall (and one unit long).
Part 2 of the wolfire blog talks specifically about field of view and dot product.
Sine, cosine, and tangent are used to relate the unit components of a vector to the vector's angle
against the coordinate +X unit vector.
To rotate stuff, you must first “translate” (or
move) the thing you wish to rotate to the coordinate origin, perform your rotation, and then move it back to the original location.
I hope this helps get you started. I know there's a lot to wrap your head around, so be patient and take the time to study the math.
Let us know where you get stuck.