Do all the network cards use the same frequency to send signals to wire?

I am comparing my cable TV wire to my network wire. In a TV cable wire, different frequencies are used by different TV channels. And since a certain channel use a fixed frequency, I think the only left way to represent different signal is with the carrier wave's amplitude.

But what about the network wire? For all the network cards with the same type, do they also use different frequencies to send signals just like TV cable? I vaguely remember that they use frequency adjustment to represent signals. So the frequency should not be a fixed one. So how did all the network cards that sharing the same medium differentiate their own signal from others?

Assuming you're asking about how the Ethernet PHY layer handles multiplexing, here's a simplified answer.

Each network consists of at least 2 stations sharing the same medium (wire). More than 2 stations sharing the wire is pretty common with Ethernet. Because they're sharing a medium, only one station can "communicate" at a time. First-come-first-served is how they decide who gets to use the medium.

To prevent the network from degrading too much due to collisions (more than 1 station sending at the same time), Ethernet uses CSMA/CD (carrier sense multiple access with collision detection) which in laymen's terms means "if no one's using the wire, try to send. If you detect a collision while sending, stop and wait a random amount of time before trying again".

Frequency on the network has to do with to the number of cycles per second, and relates more to the speed or bandwidth of the transmission.

Ethernet over twisted pair has no frequency-division multiplexing (FDM) or other frequency shifting modulation in use; each signal has full control of wire, on a single frequency. The transmission of data is carried out by voltage changes. A packet rolling down the Ethernet cable is represented as a series of voltage changes, each with a corresponding change in current. When a signal is impressed on a line (when a host sends a packet on the Ethernet), the voltage at the end of the wire must go from 0 to -2.5 volts.

Otherwise, devices from different manufacturers could not talk together and adapt their speeds/frequency to a common value if they also had to agree on a common frequency.