Centripetal vs Centrifugal co-exists?

Rotational Motion

In order to maintain a circular motion, centripetal and centrifugal forces must equal, this we all know, but the concept is unclear, where comes this centrifugal force that appose the centripetal force. In the case of a roller coaster, it is R, but rotatioanl motion based on Strings and Gravity, what is the centrifugal force?.....

Here I have a case of a roller coaster, R-g= mv^2/r at bottom, R+g = mv^2/r at top, the 3 variables change accordingly when the roller coaster moves. I always tend to think that one of it is constant that makes the whole thing conplicated.

the R + g or R - g relation is actually the centripetal force and this centripetal force changes accordingly to mv^2/r, what is the main point here, AH HA !! Gravity doesn't change according to the centrifugal force, instead, gravity is always the same depend on what angle the coaster is at. So, gravity is the priority, and then there comes the centrifugal that is confusing. The R is CREATED by the CENTRIFUGAL FORCE to balance out everything, the track act as a essential element for this R to be adjusted, increase or decrease, and the CENTRIFUGAL FORCE will ONLY exist when ROTATIONAL MOTION is present, it looks LIKE newton's 3rd law, but not that intuitive. so at the top of the circle, if the coaster wants to keep on track, the minimum available centripetal force must be overcomed, and the minimum available is present, WHEN gravity is the only centripetal force<< why? because we cannot change gravity... and Thats it.

Petal = Fugal
R+G = mv^2/r