if you see a community for the first time, it will take a while to load the last 20 posts on that community. after that, every time an update is done on the remote community, it will be pushed to yours.

idk if at least one person need to subscribe to it, or just opening it is sufficient.

add multi community aggregation to aggregate communities of the same topic on different instances.

there is an app for mobile. that also offer continuous backup of photos (like gphoto)

if they have the same hash the deduplication thing will work. if they are different quality or other stuff no. thare are plans to implement something regarding more advanced deduplication but not anything implemented at the moment.

use https://federation-checker.vercel.app/ to check is the instance you are considering is blocked by others. and what the instace blocks.

that's because i run manjaro on my main machine, all the benefits of arch + simple intsall

no, it's an app trick. not all apps allow it. it's on the roadmap to be integrated in lemmy itself btw.