Catalized problem

Good night, I have a problem with the catalysts of a Ford Explorer because it does not bring any, the data is the following:
2000 Ford Explorer XLT, Automatic transmission of 5 (5R55E), 4.0L - 6CyL
SOHC. VIN: 1FMZU73E1YZB14541.
I have taken it to several places but they don't know how many it takes, some tell me that it carries 2, another 3 and others up to 4 and the truth I have doubts.
It has 2 gray oxygen sensors at the top one on left side. And another on the right side.
It has 1 blue sensor and another canceled (bridge) at the bottom in the center.
They tell me that it carries the catalysts OBDII, but do not really know if they are carrying them are all the same or not. I have taught two different guys, one small oval that tell me goes up but I don't know what side, and a bigger one that is supposed to go in the center.
They are Magnaflow brand.
I also do not know if the 2 blue sensors are connected or not and in a way, I suppose that one before and another after the center catalyst.
Thank you very much for your time and appreciable help

Good evening, look I have a 4 liter engine and have 2 catalysts and these are co -pilot side and has 1 sensor before and 1 sensor after catalysts.