How does factory reset work? I have 2 Samsung tablets, one serves as my main device, and I would like to factory reset the other one. I don't want to lose any of my data.
What happens if I reset one of them? Do they both lose their data? How can I prevent that from happening? What can get lost?
You have luck, you can safely factory reset the secondary tablet. All the data on it will be deleted, but your main tablet will remain unaffected.
Factory reset is a device-only thing.
Androids are no different than ordinary computers, only the most important apps have the habit that they automatically backup everything to the network (typically into the cloud service of the developer company). They also like to make cloudy the border between the files on your device, and the files backed up to them.
Backup all the important data on your secondary tablet and you can make the factory reset safely.
In general, it is very useful to have backup all of your important data. Most apps does it (for example, most androids automatically uploads the photos into the google cloud), but not all of them.