Who can exactly edit them and how do I make it so only Administrators can edit them?
Admins can edit them. To monitor which admins you can go Admin CP > Users & Groups > Admin Permissions and then either Group Permissions or {User} Permissions and find "Can manage forum announcements?".
Moderators can also edit them in the Mod CP but I don't know how you can block that. I guess you could remove that section from the PHP files somehow as one option.
I'm not that good at PHP yet so I don't want to mess up the files.
I meant the template files.