Check the DDJ-Rev5 manual for the best description.
The FX is not global, but applied to either left or right channel and the decks that are active on that channel (1/3 on the left, 2/4 on the right), activated by the two levers on either side just below the DDJ-Rev5 logo.

The 6 FX buttons are used to select which effect(s) will be applied when the lever is activated. (Press more than one at the same time to activate multiple effects!)
There are up to 4 banks of 6 effects that can be saved, Banks A and B selectable by the Rev5.

From the manual:
When using Serato DJ Pro
1. FX SELECT buttons
Select an effect.
• Press the [FX2-2] button while pressing the [SHIFT] button to select Bank A, and
press the [FX2-3] button while pressing the [SHIFT] button to select Bank B.
• Turn the rotary selector while pressing the [FX SELECT] button to select an effect
type.
• Press multiple [FX SELECT] buttons at the same time to use multiple effects.
2. BEAT <,> button
Sets the number of beats for an effect.
• Press the [ <] button while pressing the [SHIFT] button to automatically set BPM
(Auto BPM).
• Press the [ >] button while pressing the [SHIFT] button to manually set BPM
according to the average time between taps (Tap mode).
3. Effect lever
Applies an effect.
— [LOCK ON]: Continues applying an effect even after you release the effect lever
leaving it in the [LOCK ON] position. Return the effect lever to the center position
to turn the effect off.
— [ON]: Applies an effect while you hold the effect lever in the [ON] position.
Release the lever to return it to the center position and turn the effect off.
4. SHIFT button
5. LEVEL/DEPTH knob
Adjusts the quantitative parameters for effects.

Thank you for your time. This is what I read too. But if I want a delay fx on deck 1/ channel 1 and at the same time a transformer fx to scratch on deck2/ channel 2 and lock the paddles on each deck BOTH channels receive delay and transform fx. This is crazy. I can't have separate FX at the same at on 2 channels? Maybe I'm missing something lol, Thanks again if you have any answers.

Ah! I see what you are saying! I missed that when I first read it. That is an interesting idea!
Yeah, there is not a per channel FX per se.

Not sure what controller would have true simultaneous and distinct FX to multiple channels. Granted I am only familiar with Rev-5 and DDJ-FLX6

Yeah, it's a pretty basic feature and huge let down that a $1K controller doesn't have it... Crazy