Its on all wheels.
its in the main brake modulator
its a complex system.
why ask this, the FSM book covers all that.
read it?
take a ASE class on brakes, that be #1
or read the wiki, that can be a free good start.
why not get the car serviced, like 99.9% of everyone does.
is there a reason to spend a month learning ABS to fix them?

https://en.wikipedia.org/wiki/Anti-lock_braking_system

first check fuses
then scan it, did you, seem not.
did you check brakes first, you know the stopping parts.
if they are bad, dont expect the magic ABS to work right,
seem logical that?
if say brakes stick and or lock/drag, the ABS will go nuts.
see? and lots more, that be just one tiny example.

Under the hood is the control module and hydraulic unit.