That depends on the game you hosted. Here is our wiki guide on save data locations. https://survivalservers.com/wiki/index.php?title=Server_Saved_Files
The mods would be added using FTP. The mod may also need to be installed on your computer as well. I would check with the mod developer for more information as not all mods are compatible with dedicated servers.
These are the only ones that we are aware of. https://survivalservers.com/wiki/index.php?title=SCUM_Server_Settings
Announcements are possible. https://survivalservers.com/wiki/index.php?title=SCUM_Admin_Commands
I do not believe it needs to be enabled. It is added in the .txt file using FTP. If you need further assist, feel free to submit a ticket so we can take a look!
Switching branches will wipe the server directory so the new files can be added. With that being said, you can restore a backup at the bottom of your control panel after you switch or manually save the save files using FTP first before switching branches.