Are packages installed by snap from https://snapcraft.io inherently safe? I heard they are sandboxed. Does that mean they can't change my file system, read my files, etc.?
Snap has several "confinements". The most popular is called "strict", which severely limits what an app can access. The app has to explicitly ask for permission to access user files, network access or processes. These apps are sandboxed and are therefore safer.
However, some apps run in "classical" confinement, which allows access to all of these things, and are therefore far less safe.