A friend of mine told me they measure with nuclear decay to tell time on satellites but I don’t understand what TF that means
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Not all satellites “tell time” on their own, many just have the time told to them by clocks on the ground. It’s only GPS satellites specifically that track time on their own. They have atomic clocks onboard, but those are the same as atomic clocks on the Earth. Are you asking how atomic clocks in general work?
Not quite. Most modern clocks use something which vibrates at a specific frequency. Computers use a digital peizo crystal which is not very consistent. Modern watches use a quartz crystal which is fairly but not perfectly consistent. Atomic clocks use the resonant frequency of charges in the energy states of cesium. This is so consistent that we have decided to define the length of a second based on it.
No nuclear decay is involved.
No clocks use nuclear decays, this is a common misconception.
Some (not all) satellites use atomic clocks: They use transitions between energy levels of atoms (not their nuclei) that can be induced when they are hit by radiation with a very specific frequency. You can measure the frequency when that happens and you know one second has to be x times the period of that radiation.
Satellites that don’t need their own atomic clocks can get their time from GPS signals, from their own less precise clocks, or from ground stations, depending on what works best for them.
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