I think this can give you more information:
https://stackoverflow.com/questions/51279711/what-does-1000-mean-in-chgrp-and-chown
This means it assigns the owner as the first linux user on that machine.

why not use my user and group?

I think because they create a folder in /opt/clearml/data/mongo_4 and the folder needs same permissions as were made in the setup

CostlyOstrich36 so why 1000:1000? My user and group are not that and so do all the otehr files I have under /opt/clearml

But I believe you can use your user/group if you prefer 🙂