how are stereo signals recorded on records and CDs

Not on two sides two channels for vinyl.
The original monaural groove was a lateral movement only. the up and down was not used.
When stereo came about, the vertical direction was used for the other channel. So the horizontal modulation became rotated 45 degrees and became one channel, then the other vertical modulation was rotated the 45 degrees and became the other channel. So the two channels are moving at 90 degrees out of geometry from each other. Both sides of the groove have vertical and horizontal information and the stylus picks this up and due to simple physics separates the two independent channels mechanically, and then coils in the cart one way or another change that motion into the two stereo channels. With the relationship, old mono records still play as two monaural channels. Pretty cool.

For CD the pits on the Cd have NO physical relation to the ones and zeros anyone can follow easily.
Music is turned into packets of ones and zeros.
Those packets are diced up, swapped around and various strange things done to allow better data loss protection and error correction. Then they are made into a CDdisc.
When the pits and land area of the CD are read in a CD player, only the CHANGE and the time matter in recreating what they mean. So the CD player takes the raw data and recodes all those changes back into a signal using rules created in the REDBOOK CD specifications. It is doing a LOT in real time. a small miracle of electronics.

In 1958, the first group of mass-produced stereo two-channel records was issued, by Audio Fidelity in the USA and Pye in Britain, using the Westrex "45/45" single-groove system. Whereas the stylus moves horizontally when reproducing a monophonic disk recording, on stereo records, the stylus moves vertically as well as horizontally. One could envision a system in which the left channel was recorded laterally, as on a monophonic recording, with the right channel information recorded with a "hill and dale" vertical motion; such systems were proposed but not adopted, due to their incompatibility with existing phono pickup designs (see below). In the Westrex system, each channel drives the cutting head at a 45-degree angle to the vertical. During playback, the combined signal is sensed by a left-channel coil mounted diagonally opposite the inner side of the groove and a right-channel coil mounted diagonally opposite the outer side of the groove.[60]

The combined stylus motion is, in terms of the vector, the sum and difference of the two stereo channels. Effectively, all horizontal stylus motion conveys the L+R sum signal, and vertical stylus motion carries the L−R difference signal. The advantages of the 45/45 system are that it has greater compatibility with monophonic recording and playback systems. A monophonic cartridge will reproduce an equal blend of the left and right channels, instead of reproducing only one channel. Conversely, a stereo cartridge reproduces the lateral grooves of monophonic recording equally through both channels, rather than one channel. Also, it gives a more balanced sound, because the two channels have equal fidelity as opposed to providing one higher-fidelity laterally recorded channel and one lower-fidelity vertically recorded channel. Overall, this approach may give higher fidelity, because the "difference" signal is usually of low power, and is thus less affected by the intrinsic distortion of "hill and dale"-style recording.
from wikipedia