Trying to understand the backend of lemmy

Hello fellow lemmings!

I'm a new lemming like many of you and I want to know a bit more about the inner workings of lemmy. So I just have a few questions

1. If I'm part of instance A and there is a community C in instance B and I want to put a comment with a picture attached.
   1. In which instance is the data (comment+picture) stored?
2. Is it possible to migrate from instance A to instance B seamlessly while taking all my data?
   1. If yes, will my username change from user@A to user@B?
   2. If the username changes, how do my previous followers keep track?
3. When I'm just viewing (not commenting) community C@B from instance A, is some amount of data (images/post text) is transferred from B to A?
   1. If yes, is it kept for sometime or permanently?
4. If I've subscribed to 100s of communities in different instances, how much will it impact my instance?
5. Consider the link to this post lemmy.ml/post/1223070 and lemm.ee/post/24869 which basically references this post from different instances.
   1. Is there any difference in the way people see it?
   2. Will there be a sync issue when people try commenting on it?

These are just some questions that came to my mind. I'll post more in the comments as they come.