Simple answer is it does not really matter, everything is connected and most things can be reached from anywhere.

Slightly more complex answer: each server instance (e.g. lemmy.ml, lemmy.world, lemm.ee and any others) has its own rules and regulations, dictated by the individual admin(s). You should read them to know what the instance is about, its general philosophy, and if it is a good fit for you. From this also follows the fact that your instance admins can choose to disconnect from other server instances - in that case you can not reach anything from the other instance. An example is many nsfw-free instances disconnecting (‘defederating’) from porn instances like lemmynsfw. Or disconnecting from right-wing troll instances.

So, the general answer is it does not matter much, but there is some fine print to it you should be aware of.

Can I ask which one you have and does it work well with gadgetbridge?

I have been eyeing a watch from them for a while now but gadgetbridge.org seems to only list two compatible watches (or I misunderstand the Fossil naming scheme).