They are plain text files known as "Sampled Trajectories" and are documented at https://en.wikibooks.org/wiki/Celestia/Trajectories#SampledTrajectory
JPL's Horizons ephemeris server can provide xyz files for objects in the solar system, although you do have to edit the results somewhat to make them compatible with Celestia. Use its telnet interface for the most flexibility or use its e'mail interface and edit this template message: horizons-template.txt when asking for many xyz samples.
Calculating xyz or xyzv files from scratch requires the use of appropriate software. GMAT can do this, for example. See viewtopic.php?f=4&t=11410
How to make a XYZ orbit?
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