Just to make sure we’re being clear, the only way to add new zwave/zigbee devices to your account is through the official mobile app.
There is an official web interface to your account, but you can’t actually see The details of any of your rules or scenes there, just your devices. And again, you can’t add new physical devices to your account that way.
When it comes to setting up rules, webcore is available.
When it comes to operating your devices after they have been added to your account, that is toggling them on or off or changing the dim level, you can use a dashboard like ActionTiles.
But for reasons known only to Samsung, you can’t initiate the protocol that adds a zigbee or zwave device to your account except through the mobile app.
SmartThings is a very powerful, flexible system, but the UI just isn’t very intuitive or easily discoverable. You can always contact support or come here to the forum and ask if you can’t figure out how to do something. Trust me, we’ve all been there.