(I’ve moved this to smart app ideas because this kind of logic is actually run in the smart things cloud using a smart app or another automation, the SmartThings mobile app is just the UI to the system.)
The short answer, as you’ve already noted, is to use core. It can likely do every combination of if so you can imagine, and a few more. And there are many community members who will be happy to help you set up your rules.
The longer answer if you don’t want to use core is that there is probably a way to do it in SmartThings but it depends on each individual use case that you are trying to solve.
Smartthings is flexible and versatile, but one thing it is definitely not as consistent in the way that features are exposed through various options. Nor is it consistent in what’s an “and” and what a “or.”
For example, there was just a question yesterday from a community member who wanted to have different types of notifications for the official smart home monitor feature depending on whether the armed state was “armed away” or “armed Home.”
It turns out that if you are using the armed states, there’s only one place to set up the notifications, so they end up applying the same way to all of the armed states.
But if you use the official SHM feature and select “custom” alerts, then you can set up a unique notification profile.
That distinction isn’t easy to find. It’s not all that easy to understand. But it does mean this particular community member can do what they want to get done, just not the way they were trying to do it.
Just another random example: a smart lighting automation can be triggered based on a mode change. But not an SHM armed state change. Core could be triggered on either or both or an exclusive or. 
So I think it’s likely that there will be a way to get whatever it is you’re trying to do with the motion sensor accomplished without using core. I just also think that at this point it’s probably going to be a lot more straightforward to use core when you have stacked conditionals.