[Cr3aTor]

Depending on the version of ios the device has you could jailbreak it and then install some keylogger or spyware or you could make a fake app with you then can install using a something like cydia impactor.

I believe the way to unlock a new device is to keep trying to unlock it manually because if the device has ios 12 then a grey key won't work which was
used by police to until and will be used until no one has ios 11.

With spying on snapchat conversation goes with what i said first but you could do it easier by making a modded snapchat and making them install it.

^^This is basically how you can do it, however you are going to need physical access to the phone or a form of social engineering in order to jailbreak the phone.
After that, it's pretty much done, you just need to make a backdoored version of snapchat or any innocent app and apply again a form of SE/physical access to install it.
You can go a little further and once you have access to the phone install other apps made by you that are hidden for the user and make a pretty web interface to get all you need.

I shall say that you should be carefull with those kind of actions, since they can cause you a lot of trouble if executed incorrectly. So if you want just to spy on a friend or something like that, I advise you to not do it.  

What about Wifi router if you have admin rights? (Did not understand what you meant)

I think he is asking what you can do if you have access to the router (correct me if I'm wrong). By having control over the router, you can pretty much just do what is on the web interface of it. An example might be to change the DNS server. It is your choice what to change in your advantage, obviously based on the goal you want to accomplish.