Why are satanism and freemasonry so intertwined?
I don't think there is any relation whatsoever. Why do you think they are intertwined?
One side of Freemasonry requires religious belief in God, creator of the universe. The other side doesn't require any religious belief but leaves it to each individual what they want to believe or not. None expect candidates to shun God and believe in Satan, let alone worship Satan.
If it is a misconception, what is it about either one that you think would lead to that confusion would you say?
I have no idea why you would think this.
If you would explain why you think Freemasonry has anything to do with Satanism, we can talk about what led to that confusion.
The knights templars, skull n bones, illuminati- are these all branches of freemasonry?
Absolutely not. You can Google Masonic obediences ("branches") to see what they are.
Knights Templars were religious and military order that predate Freemasonry and was exterminated during the Templar trials of 1307-1310, if I recall correctly. There was probably an overlap around the Middle Ages, because some of the allegoric stories in Masonic rituals feature Knights Templar.
Da Vinci Code and Foucault's Pendulum have interesting theories on this subject, but as far as I know they are fiction.
When you say rituals, could you give an example of one.
I couldn't tell you about them in detail, but think of rituals like ceremonies - scripted words and actions for opening and closing ofa meeting, initiations, passing to each new degree, etc.
Also, do you understand the significance behind each and everyone?
Yes, I understand the meaning and significance of the rituals I have seen and performed so far.