4. There are the following main builders which allow to move things around:

- Pages - to add/remove/edit/rearrange blocks on different pages
- Menus - to add/remove/edit/rearrange menu items
- Forms -  to add/remove/edit/rearrange different form fields
- Permissions - to change user's permissions by levels

5. Sorry, it's not clear what you mean. We have different communication modules:

- Jot - messenger real time messaging between 2 or more people
- Conversations - not real time private messaging between two or more people
- Rocket.Chat integration - usually used for public chats, but there is functionality for closed channels and private chats

6. All customizations should be untouched during upgrades, however some changes may still affect your customiation in rare cases, but data is untouched and it should be easy to apply it again. If you create separate template, in some cases small changes are required in custom templates, like adding some new files.